usbtv: split core and video implementation

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-by: Lubomir Rintel <lkundrak@v3.sk>
usbtv: add audio support

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Tested-by: Lubomir Rintel <lkundrak@v3.sk>