s32k3xx:Serial refactor out tx dma semaphore

Fixes stuttering output.

  The use of the semaphore was causing blocking
  on non blocking callers. This ensured that
  the TX DAM would be restated, but when it
  was switched to trywait in 8362e314, it left
  data in the xmit queue unsent.

  This solution removes the semaphore and restart
  the DMA on completion if there is more data in
  the xmit queue to be sent.

arch/arm/src/s32k3xx/s32k3xx_serial.c

@@ -1332,7 +1332,6 @@ struct s32k3xx_uart_s
 #ifdef SERIAL_HAVE_TXDMA
   const unsigned int dma_txreqsrc;  /* DMAMUX source of TX DMA request */
   DMACH_HANDLE       txdma;         /* currently-open transmit DMA stream */
-  sem_t              txdmasem;      /* Indicate TX DMA completion */
 #endif
 
   /* RX DMA state */
@@ -2909,8 +2908,6 @@ static int s32k3xx_dma_setup(struct uart_dev_s *dev)
             {
               return -EBUSY;
             }
-
-          nxsem_init(&priv->txdmasem, 0, 1);
         }
 
       /* Enable Tx DMA for the UART */
@@ -3112,7 +3109,6 @@ static void s32k3xx_dma_shutdown(struct uart_dev_s *dev)
 
       s32k3xx_dmach_free(priv->txdma);
       priv->txdma = NULL;
-      nxsem_destroy(&priv->txdmasem);
     }
 #endif
 }
@@ -3948,9 +3944,9 @@ static void s32k3xx_dma_txcallback(DMACH_HANDLE handle, void *arg, bool done,
 
   uart_xmitchars_done(&priv->dev);
 
-  /* Release waiter */
+  /* Send more data if available */
 
-  nxsem_post(&priv->txdmasem);
+  s32k3xx_dma_txavailable(&priv->dev);
 }
 #endif
 
@@ -3969,9 +3965,7 @@ static void s32k3xx_dma_txavailable(struct uart_dev_s *dev)
 
   /* Only send when the DMA is idle */
 
-  int rv = nxsem_trywait(&priv->txdmasem);
-
-  if (rv == OK)
+  if (s32k3xx_dmach_idle(priv->txdma) == 0)
     {
       uart_xmitchars_dma(dev);
     }