V4L/DVB (7782): pvrusb2: Driver is no longer experimental

This driver has been in-kernel and reasonably stable for well over a
year.  It is in a stable form and is known to work well.  Remove its
experimental status.

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