V4L/DVB (7696): pvrusb2: state control tweak

Don't trigger a pathway state change if it's already been triggered
(eliminates some wasted processing and some debug output noise)

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