fix: possible endless loop in case some error flag is set but RXNE is not

Fixes #3482

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

@@ -557,15 +557,15 @@ void stm32_usart_isr(const stm32_usart_t* usart, etx_serial_callbacks_t* cb)
     // Drain RX
     while (status & (LL_USART_SR_RXNE | USART_FLAG_ERRORS)) {
 
+      // This will clear the RXNE/error bits in USART_SR register
+      uint8_t data = LL_USART_ReadReg(usart->USARTx, DR);
+
       if (status & USART_FLAG_ERRORS) {
         if (cb->on_error)
           cb->on_error();
       }
 
       if (status & LL_USART_SR_RXNE) {
-        // This will clear the RXNE bit in USART_DR register
-        uint8_t data = LL_USART_ReadReg(usart->USARTx, DR);
-
         if (cb->on_receive)
           cb->on_receive(data);
       }