gpio: mockup: parse the module params in init, not probe

If the module parameters are invalid, we should bail out from the init
function instead of detecting it during the device probe. That way we
don't even allow the user to load the module if we don't accept the
arguments.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>