stm32h7/serial: Do not wait on TXDMA semaphore

If using flow control with a high CTS the thread may be blocked forever
on the second transmit attempt due to waiting on the txdma semaphore.
The calling thread can then never make progress and release any
resources it has taken, thus may cause a deadlock in other parts of the
system.

The implementation differs in behavior from interrupt-driven TX and the
STM32F7 TXDMA . It should not implicitly wait on a taken semaphore but
return immediately and let the upper layers decide on what to do next.

arch/arm/src/stm32h7/stm32_serial.c

@@ -3369,9 +3369,11 @@ static void up_dma_txavailable(struct uart_dev_s *dev)
 
   /* Only send when the DMA is idle */
 
-  nxsem_wait(&priv->txdmasem);
-
-  uart_xmitchars_dma(dev);
+  int rv = nxsem_trywait(&priv->txdmasem);
+  if (rv == OK)
+    {
+      uart_xmitchars_dma(dev);
+    }
 }
 #endif