[media] Re-write the s921 frontend

On our tests with Leadership ISDBT, the s921 frontend were not work. As its
design contained some weird things, it ended to be easier to just re-write
it, getting another frontend as an example (cx24123).

As the old s921 driver weren't used, there's no regression. Some info from
the old frontend were used as a way to double check the behavior that were
noticed on the USB dumps retrieved from Leadership driver.

Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>