pinctrl: ns2: fix return value check in ns2_pinmux_probe()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>