gpio: vr41xx: fix up errorpath on probe()

The driver was not checking the return value from gpiochip_add()
properly, so add a bail-out check.

Reported-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>