[media] em28xx: prevent registering wrong interfaces for audio-only

A few devices (em2860) use a separate interface for audio only
Audio Vendor Class USB. That interface should not be used by
Remote Controller, Analog TV or Digital TV.

Prevents initializing all non-audio extensions for the audio
only interface.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>