gpio: fix gpio leak in gpiochip_add error path

Make sure to free any hogged gpios on errors in gpiochip_add.

Also move all forward declarations to the top of the file.

Fixes: f625d460

 ("gpio: add GPIO hogging mechanism")
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>