regulator: vexpress: Use PTR_ERR_OR_ZERO() to simplify code

Fixes coccicheck warning:

drivers/regulator/vexpress-regulator.c:78:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Link: https://lore.kernel.org/r/1574074762-34629-1-git-send-email-zhengbin13@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>