V4L/DVB (11206): pvrusb2: Add sub-device for demod

Forgot to include the tda9887 component when moving to v4l2-subdev.  I
got fooled because its name is "tuner", the same as the tuner module.
Silly me.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>