Use HAL coding style for function naming

Author: LMESTM

targets/TARGET_STM/serial_api.c

@@ -694,7 +694,7 @@ int8_t get_uart_index(UARTName uart_name)
  *  yet. Returns 1 if there is at least 1 serial instance with ongoing ransfer
  *  0 otherwise.
  */
-int serial_IsTxOngoing(void) {
+int serial_is_tx_ongoing(void) {
     int TxOngoing = 0;
 
 #if defined(USART1_BASE)

targets/TARGET_STM/sleep.c

@@ -157,7 +157,7 @@ void hal_sleep(void)
     core_util_critical_section_exit();
 }
 
-extern int serial_IsTxOngoing(void);
+extern int serial_is_tx_ongoing(void);
 
 void hal_deepsleep(void)
 {
@@ -167,7 +167,7 @@ void hal_deepsleep(void)
      *  This is tracked in mbed issue 4408.
      *  For now, we're checking all Serial HW FIFO. If any transfer is ongoing
      *  we're not entering deep sleep and returning immediately. */
-    if(serial_IsTxOngoing()) {
+    if(serial_is_tx_ongoing()) {
         return;
     }