V4L/DVB: pvrusb2: Fix minor internal array allocation

pvrusb2: Need one extra attribute slot allocated so that worst case still has a
trailing null pointer.  This wasn't causing visible symptoms; it was
found through inspection while investigating other issues.

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