[media] v4l: Event documentation fixes

Constify event type constants and correct motion detection event number
(it's 6, not 5).

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>