media: ov2680: Wait for endpoint fwnode before continuing with probe()

Defer probe() until the endpoint fwnode is available. This is necessary
on ACPI platforms where the bridge code creating the fwnodes may also e.g.
set the "clock-frequency" device property and add GPIO mappings.

Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>