pinctrl: lantiq: remove bogus of_gpio_chip_add

Remove bogus call to of_gpiochip_add (and of_gpio_chip remove in error
path) which is also called when adding the gpio chip.

This prevents adding the same pinctrl range twice.

Fixes: 3f8c50c9 ("OF: pinctrl: MIPS: lantiq: implement lantiq/xway
pinctrl support")

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>