V4L/DVB (4661): Pvrusb2: improve 24XXX config option description

The CONFIG_VIDEO_PVRUSB2_24XXX is not nearly as "experimental" as the
description suggests.  So refine the description to better match reality.

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