serial: imx: simplify check that prevents starting PIO when DMA is in use

The original code looks as follows:

	if (sport->dma_is_enabled) {
		... make sure TX DMA is running, i.e. .dma_is_txing = 1
	}

	if (sport->dma_is_txing)
		return;

As .dma_is_txing can only be true if .dma_is_enabled is, the return can
go at the end of the first if body without an additional check.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -504,10 +504,9 @@ static inline void imx_transmit_buffer(struct imx_port *sport)
 			imx_uart_writel(sport, temp, UCR1);
 			imx_dma_tx(sport);
 		}
-	}
 
-	if (sport->dma_is_txing)
 		return;
+	}
 
 	while (!uart_circ_empty(xmit) &&
 	       !(imx_uart_readl(sport, uts_reg(sport)) & UTS_TXFULL)) {