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>