gpio: gpio-mm: Mask read inputs for get_multiple

This patch masks the read inputs with the word mask in order to ensure
only requested input states are returned in the bits array.

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>