Merge pull request #7075 from donatieng/fix_cordio_h4_serial

BLE: Replace Serial with RawSerial in Cordio H4 Transport Driver

features/FEATURE_BLE/targets/TARGET_CORDIO/driver/H4TransportDriver.h

@@ -69,7 +69,11 @@ public:
 private:
     void on_controller_irq();
 
-    Serial uart;
+    // Use RawSerial as opposed to Serial as we don't require the locking primitives
+    // provided by the Serial class (access to the UART should be exclusive to this driver)
+    // Furthermore, we access the peripheral in interrupt context which would clash
+    // with Serial's locking facilities
+    RawSerial uart;
     PinName cts;
     PinName rts;
 };