gpio: mlxbf2: fix return value check in mlxbf2_gpio_get_lock_res()

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

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200427110829.154785-1-weiyongjun1@huawei.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>