serial: imx: ensure UCR3 and UFCR are setup correctly

Commit e61c38d8 ("serial: imx: setup DCEDTE early and ensure DCD and
RI irqs to be off") has a flaw: While UCR3 and UFCR were modified using
read-modify-write before it switched to write register values
independent of the previous state. That's a good idea in principle (and
that's why I did it) but needs more care.

This patch reinstates read-modify-write for UFCR and for UCR3 ensures
that RXDMUXSEL and ADNIMP are set for post imx1.

Fixes: e61c38d8 ("serial: imx: setup DCEDTE early and ensure DCD and RI irqs to be off")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Mika Penttilä <mika.penttila@nextfour.com>
Tested-by: Mika Penttilä <mika.penttila@nextfour.com>
Acked-by: Steve Twiss <stwiss.opensource@diasemi.com>
Tested-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>