pinctrl: mediatek: Consistently use the BIT() macro

The mediatek pinctrl driver uses the BIT() macro instead of open
coding 1 << n in all but one place. Replace this occurrence with
the BIT() macro for consistency.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>