media: i2c: ds90ub9x3: Fix sub-device matching

Commit 1029939b ("media: v4l: async: Simplify async sub-device fwnode
matching") recently changed how async sub-device matching works. This
breaks the UB9x3 drivers, as they set the subdev.fwnode to an endpoint.
Afaiu, the fix is simply to not set subdev.fwnode at all.

Fixes: 1029939b ("media: v4l: async: Simplify async sub-device fwnode matching")
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Cc: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>