gpio: gpiolib: Normalize return code variable name

It is confusing to name return variables mixedly "status",
"err" or "ret". I just changed them all to "ret", by personal
preference, to lower cognitive stress.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20190716091145.8235-1-linus.walleij@linaro.org