pinctrl: st: fix return value check

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.
The function syscon_regmap_lookup_by_phandle() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Mark Brown <broonie@linaro.org>