[media] mb86a20s: improve error handling at get_frontend

The read/write errors are not handled well on get_frontend. Fix it,
by letting the frontend cached values to represent the DVB properties
that were successfully retrieved.
While here, use "c" for dtv_frontend_properties cache, instead of
"p", as this is more common.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>