pinctrl: mediatek: moore: fix return value check in mtk_moore_pinctrl_probe()

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

Fixes: b7d7f9ee ("pinctrl: mediatek: extend struct mtk_pin_desc which per-pin driver depends on")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>