Take note of which sysfs files / groups that the pvrusb2 successfully
creates and only delete those specific items when tearing things
down.  (Previously we just blindly tore everything down every time.)
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>