[media] pwc: v4l2-compliance fixes

- add device_caps
- set colorspace
- s_parm should support a fps of 0 (== reset to nominal fps)

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