[media] ov2640: print error if devm_*_optional*() fails

devm_gpiod_get_optional() can return -ENOSYS if GPIOLIB is
disabled, causing probe to fail. Warn the user if this
happens.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>