gpiolib: use gpiochip_get_desc() where applicable

Whenever retrieving a descriptor from a gpiochip: use the provided
helper which checks for errors.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Link: https://lore.kernel.org/r/20200219094702.6463-1-brgl@bgdev.pl


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