V4L/DVB (7298): pvrusb2: Account for dtv choice (a bit) in v4l2 implementation

The v4l2 implementation in pvru2b2 must produce a sane answer when
asked, when the input choice is set to dtv.

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