pinctrl: ralink: rt2880: use 'PTR_ERR_OR_ZERO'

 Avoid some boilerplate code using 'PTR_ERR_OR_ZERO'
 in probe function.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201213161721.6514-8-sergio.paracuellos@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>