V4L/DVB (4663): Pvrusb2: Get rid of private global context array brain damage

A previous attempt to deal with the upcoming loss of
video_set_drvdata() and video_get_drvdata() resulted in logic which
causes a circular locking dependency - also known as a deadlock.  This
changeset attacks the problem in a different manner, using a technique
that no longer requires the problematic mutex (or that private global
array either).

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