pinctrl: artpec6: Fix return value check in artpec6_pmx_probe()

In case of error, the function pinctrl_register() returns
ERR_PTR() not NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>