[media] DocBook media: fix number of bits filled with zeros for SRGBB12

The total number of bits is 16 (12+4). Fix it.

Signed-off-by: Andreas Weber <andy.weber.aw@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>