[media] pvrusb2: Minor VBI tweak to help potential CC support

pvrusb2: Declare closed-caption setup for line 21 - this is needed for
sliced VBI capture support.  (However none of that works right now
anyway.)

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