serial: imx: disable TDMAEN in imx_flush_buffer()

Terminating the DMA, make sure the interrupt is disabled, too.
This fixes random kernel Oops due to dma_tx_call() called for
invalid transmissions.

If we disable the TDMAEN, make sure it's enabled again if a TX
DMA is started.

Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>