gpio: Get rid of _prefix and __prefixes

The arbitrarily marking of a function with _ or __ is taking to mean
"perform some inner core of the caller" or something like that. At other
times, this syntax has a totally different meaning.

I don't like this since it is unambious and unhelpful to people reading
the code, so replace it with _commit() suffixes.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>