M28: Fix OB1 bug in GPIO driver

This patch fixes a small off-by-one bug in the GPIO driver for the mxs platform that allowed the selection gpio pins of one bank more than the SoC actually has.

Signed-off-by: Robert Deliën <robert at delien.nl>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>