V4L/DVB (5057): Pvrusb2: Stream configuration cleanups

Clean up and tighten logic involving stream configuration.  This
mainly involves changes to pvrusb2-v4l2.c, where we better clarify how
we use the stream configuration enum and implement a cleaner means to
control streaming for a given device node.

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