gpio: clean up gpiochip_remove

Clean up gpiochip_remove somewhat and only output warning about removing
chip with GPIOs requested once.

Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>