mxc/gpio: make _set_value work with values != 0/1

Documentation/gpio.txt specifies that the value argument to
gpio_set_value() should be handled as a boolean (E.G. != 0 is high),
so use the same logic as in _set_direction().

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>