[media] Rename set_frontend fops to set_frontend_legacy

Passing DVBv3 parameters to set_frontend is not fun, as the
core doesn't have any way to know if the driver is using the
v3 or v5 parameters. So, rename the callback and add a new
one to allow distinguish between a mixed v3/v5 paramenter call
from a pure v5 call.

After having all frontends to use the new way, the legacy
call can be removed.

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