V4L/DVB (5051): Pvrusb2: Better radio versus tv frequency handling

Separate track radio versus tv frequency so that when we switch modes
we can also switch to a sane frequency appropriate for the mode.  Also
implement logic to automate mode switching in certain cases.

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