[media] cx25821: remove references to subdevices that aren't there

This driver does not have subdevices, so why call subdev ops? After
removing that it became apparent that only Composite is supported as
input, so remove also any reference to other inputs.

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