media: ov9640: Don't check for NULL on devm_gpiod_get return values

devm_gpiod_get never returns NULL; therefore it's not necessary to check
for that. PTR_ERR(NULL) also yields zero, which is confusing to smatch.

Reported-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>