This makes the driver work with the new check in
v4l2_async_register_subdev() that was introduced recently in 6.6-rc1.
Without this change, probe fails with:

ov8858 1-0036: Detected OV8858 sensor, revision 0xb2
ov8858 1-0036: sub-device fwnode is an endpoint!
ov8858 1-0036: v4l2 async register subdev failed
ov8858: probe of 1-0036 failed with error -22

This also simplifies the driver a bit.
Signed-off-by: Ondrej Jirman <megi@xff.cz>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>