gpio: mt7621: Edit to preferred syntax

This fixes some syntactic nits that makes the GPIO maintainer
happier. It is way easier to show by example and do it myself
than to try to explain it with comments. It's just my personal
taste of minimalism.

Cc: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Cc: NeilBrown <neil@brown.name>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>