[media] dvb-core: add support for a DVBv5 get_frontend() callback

Creates a DVBv5 get_frontend call, renaming the DVBv3 one to
get_frontend_legacy(), while not all frontends are converted.

After the conversion for all drivers, get_frontend_legacy()
will be removed.

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