pinctrl: mediatek: moore: check if pin_desc is valid before use

Certain SoC are missing the middle part gpios in consecutive pins,
it's better to check if mtk_pin_desc is a valid pin for the extensibility

Signed-off-by: Sam Shih <sam.shih@mediatek.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Link: https://lore.kernel.org/r/20210914085137.31761-5-sam.shih@mediatek.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>