serial: imx: Prevent TX buffer PIO write when a DMA has been started

serial: imx: Prevent TX buffer PIO write when a DMA has been started

Function imx_transmit_buffer starts a TX DMA if DMA is enabled, since
commit 91a1a909f921 ("serial: imx: Support sw flow control in DMA mode").
It also carries on and attempts to write the same TX buffer using PIO.
This results in TX data corruption and double-incrementing xmit->tail
with the knock-on effect of tail passing head and a page of garbage
being sent out.

This seems to be triggered mostly when using RS485 half duplex on SMP
systems, but is probably not limited to just those.

Tested locally on an i.MX6Q with an RS485 half duplex transceiver on
UART3, and also by Clemens Gruber.

Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Ian Jamison <ian.dev@arkver.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>