[media] em28xx: simplify usb audio class handling

As far as we know devices can either have audio class or vendor class
usb interfaces but not both at the same time. Even if both interface
types could be provided by devices at the same time, the current code is
totally broken for that case.

So clean up and simplify the usb audio class handling by replacing
fields "has_audio_class" (device has usb audio class compliant
interface) and "has_alsa_audio" (device has vendor audio interface) in
struct em28xx with a single enum em28xx_usb_audio_type.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>