[media] dt3155v4l: fix format handling

Fix various v4l2-compliance issues regarding format handling.

Main problem was a missing colorspace value and incorrect format
checks. This driver supports a single format only.

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