Fix can_read() return value for STM32 boards

targets/TARGET_STM/TARGET_STM32F0/can_api.c

@@ -235,6 +235,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32F1/can_api.c

@@ -238,6 +238,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32F2/can_api.c

@@ -248,6 +248,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);
 
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32F3/can_api.c

@@ -238,6 +238,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32F4/can_api.c

@@ -251,6 +251,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32F7/can_api.c

@@ -251,6 +251,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {

targets/TARGET_STM/TARGET_STM32L4/can_api.c

@@ -238,6 +238,12 @@ int can_read(can_t *obj, CAN_Message *msg, int handle)
 
     CAN_TypeDef *can = (CAN_TypeDef *)(obj->can);    
     
+    // check FPM0 which holds the pending message count in FIFO 0
+    // if no message is pending, return 0
+    if ((can->RF0R & CAN_RF0R_FMP0) == 0) {
+        return 0;
+    }
+
     /* Get the Id */
     msg->format = (CANFormat)((uint8_t)0x04 & can->sFIFOMailBox[handle].RIR);
     if (!msg->format) {