V4L/DVB (9185): S2API: Ensure we have a reasonable ROLLOFF default

Non-initialised cache values get a reasonble default.

Signed-off-by: Darron Broad <darron@kewl.org>
Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>