regulator: stm32-pwr: Fix return value check in stm32_pwr_regulator_probe()

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

Fixes: 6cdae817 ("regulator: Add support for stm32 power regulators")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>