Input: imx_keypad - only set enabled columns to open-drain

In imx_keypad_inhibit(), all 8 columns were set to open-drain, in
contrast to the rest of the driver, where only the enabled columns
are modified/used.

Contrary to the normal expectation, this also affects column I/Os not
even mapped via IOMUX to the KPP hardware module but used as a GPIO.
Therefore only init enabled columns to open-drain and leave all others
with their default reset value of 0, i.e. totem-pole.

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>