media: imx7-mipi-csis: Add a check for devm_regulator_get

devm_regulator_get may return an error but mipi_csis_phy_init misses
a check for it.
This may lead to problems when regulator_set_voltage uses the unchecked
pointer.
This patch adds a check for devm_regulator_get to avoid potential risk.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>