media: i2c: ov5640: Check for devm_gpiod_get_optional() error

The power down and reset GPIO are optional, but the return value
from devm_gpiod_get_optional() needs to be checked and propagated
in the case of error, so that probe deferral can work.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>