pinctrl: mediatek: mtk-common: initialize unmask

cppcheck detected an uninitialized variable:

[drivers/pinctrl/mediatek/pinctrl-mtk-common.c:897]:
  (error) Uninitialized variable: unmask

unmask should be initialized to zero to ensure unmasking
only occurs if a previous mask occurred. The current situation
is that the unmask variable could contain any random garbage
causing random unexpected unmasking.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>