The function can return negative values, so its result should
be assigned to signed variable.

The problem has been detected using coccinelle semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci.

Fixes: 59ee9c96 ('pinctrl: mediatek: Add gpio_request_enable support')
Signed-off-by: NAndrzej Hajda <a.hajda@samsung.com>
Acked-by: NHongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>