V4L/DVB (7309): pvrusb2: Enhance core logic to also control digital streaming

This is a major pvrusb2 change.  The driver core has an algorithm that
is used to cleanly sequence the changes needed to enable / disable
video streaming.  The algorithm had originally been written for analog
streaming, but when in digital mode the pipeline is considerably

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>