[media] bttv: rename field 'audio' in struct 'bttv' to 'audio_input'

'audio_input' better describes the meaning of this field.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>