[media] cx88: remove fmt from the buffer struct

This is a duplicate of dev->fmt and can be removed. As a consequence a
lot of tests that check if the format has changed midstream can be
removed as well: the format cannot change midstream, so this is a bogus
check.

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