V4L/DVB (7709): pvrusb2: New device attribute for encoder usage in digital mode

Some tuners seem to not work in digital mode unless the encoder is
healthy.  Implement a device attribute to represent this flag and
modify the core state machines to enforce this requirement.

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