gpio: gpio-altera: Remove gpiochip on probe failure.

On failure to setup the irq altera_gpio_probe would return an error
but not go to cleanup. This resulted in kernel fault
"Unable to handle kernel paging request at virtual address xxxxxxxx"
later on in of_gpiochip_find_and_xlate.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>