gpiolib: make comment consistent with code

Commit f4d566a8a0e6 ("gpio: change member .dev to .parent") changes
member of gpiochip from .dev to .parent. Update the corresponding
comment.

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>