serial: imx: always wake up the processes in the TX callback

The current code only wakes up the processes when the circle
buffer has less data then the WAKEUP_CHARS.

But sometimes, the circle buffer may has data more then the WAKEUP_CHARS,
in such case, the processes will hang.

This patch makes it always wakes up the processes in the TX callback.
Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -496,8 +496,7 @@ static void dma_tx_callback(void *data)
 
 	dev_dbg(sport->port.dev, "we finish the TX DMA.\n");
 
-	if (uart_circ_chars_pending(xmit) < WAKEUP_CHARS)
-		uart_write_wakeup(&sport->port);
+	uart_write_wakeup(&sport->port);
 
 	if (waitqueue_active(&sport->dma_wait)) {
 		wake_up(&sport->dma_wait);