V4L/DVB (7710): pvrusb2: Implement critical digital streaming quirk for onair devices

Implement timed measurement of encoder operation for the first time it
is run.  This allows the driver to note when the encoder has been run
successfully for at least 1/4 second.  On top of that implement
various bits to ensure that the encoder has been run once before
digital streaming for OnAir devices.  This is done via several core
state machine tweaks.

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