Consistently use "gc" for "struct gpio *" variables.

This follows the spirit of commit a0b66a73 ("gpio: Rename
variable in core APIs").
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200424141517.11582-1-geert+renesas@glider.beSigned-off-by: Linus Walleij <linus.walleij@linaro.org>