[media] v4l2-async: Use proper list head for iteration over registered subdevs

This fixes regression introduced in commit b426b3a6
[media] V4L: Merge struct v4l2_async_subdev_list with struct v4l2_subdev

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>