mfd: Ensure sm501 GPIO pin mode is GPIO when configured

When setting an GPIO to either input or output, we
should ensure that the pin configuration elsewhere
in the chip is set to GPIO in-case the initial
setup has not been done correctly.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>