fix(radio): prevent starting another USART send when one is already in progress (#6610)

radio/src/targets/common/arm/stm32/stm32_usart_driver.cpp

@@ -560,6 +560,12 @@ void stm32_usart_send_byte(const stm32_usart_t* usart, uint8_t byte)
 
 void stm32_usart_send_buffer(const stm32_usart_t* usart, const uint8_t * data, uint32_t size)
 {
+
+  if (!stm32_usart_tx_completed(usart)) {
+    // Transmission is already in progress, abort
+    return;
+  }
+
   _half_duplex_output(usart);
 
   if (usart->txDMA) {