serial: imx: Fix handling of TC irq in combination with DMA

commit 18665414 upstream.

When using RS485 half duplex the Transmitter Complete irq is needed to
determine the moment when the transmitter can be disabled. When using
DMA this irq must only be enabled when DMA has completed to transfer all
data. Otherwise the CPU might busily trigger this irq which is not
properly handled and so the also pending irq for the DMA transfer cannot
trigger.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[Backport to v4.14]
Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>