[media] adv7511: fix incorrect bit offset

The quantization bits are in bits 7-6, not 7-4, so shift by 6 instead of 4.

This bug is caused by a typo in the adv7511 datasheet.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>