extcon: ptn5150: Fix return value check in ptn5150_i2c_probe()

In case of error, the function devm_gpiod_get() 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 <weiyongjun1@huawei.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>