V4L/DVB (9180): S2API: Added support for DTV_CODE_RATE_HP/LP

Reports from users that using the new API for tuning DTV was failing,
and the cache was missing some essential items.

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