serial: imx: drop members from driver data that are only used during probe

There is no benefit in keeping this information in RAM when it's not
used any more, so better use function local variables instead.

These members are unused since c0d1c6b0

 ("serial: imx: Fix the
reporting of interrupts")

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>