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: NJohan Hovold <johan@kernel.org>
Reviewed-by: NAlexandre Courbot <acourbot@nvidia.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>