devm_gpiod_get_optional() can return -ENOSYS if GPIOLIB is
disabled, causing probe to fail. Warn the user if this
happens.
Acked-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>