V4L/DVB (6706): pvrusb2: Remove use of volatile in command sequencer

pvrusb2: Remove use of volatile for command sequencer; these variables
are set by interrupt-context code and we check their state in such a
manner that there should be no race conditions.  This had originally
been done out of paranoia, and I have since been convinced that the
paranoia is not required.

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