STM: SPI: Initialize Rx in spi_master_write

In case Rx is not initiliased its content might be random from stack.
This is causing problem in case of 8 bits read only as the left-side 8 bits
of the returned int value may contain this random byte data. This was for
instance detected when using SDFileSystem Lib.

targets/TARGET_STM/stm_spi_api.c

@@ -344,7 +344,8 @@ static inline int ssp_busy(spi_t *obj)
 
 int spi_master_write(spi_t *obj, int value)
 {
-    uint16_t size, Rx, ret;
+    uint16_t size, ret;
+    int Rx = 0;
     struct spi_s *spiobj = SPI_S(obj);
     SPI_HandleTypeDef *handle = &(spiobj->handle);