V4L/DVB (7702): pvrusb2: Rework USB streaming start/stop execution

The commands to start / stop USB streaming for an analog device are
fairly standard, owing to the fact that all supported devices
apparently started from the same common reference design.  However
with digital mode, the commands seem to vary by vendor.  This change
makes that variance more explicit.  It also cleans up a related
problem for OnAir devices which prevented digital mode from working at
all.

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