[media] em28xx: get rid of field has_audio in struct em28xx_audio_mode

Field has_audio in struct em28xx_audio_mode is used together with value
EM28XX_NO_AC97 of field ac97 to determine the internal type of audio
(none/i2s/ac97). This makes the code difficult to understand:

  !audio_mode.has_audio && audio_mode.ac97 == EM28XX_NO_AC97 => no audio
  !audio_mode.has_audio && audio_mode.ac97 != EM28XX_NO_AC97 => BUG
  audio_mode.has_audio  && audio_mode.ac97 == EM28XX_NO_AC97 => AC97 audio
  audio_mode.has_audio  && audio_mode.ac97 != EM28XX_NO_AC97 => I2S audio

Simplify the whole thing by introducing an enum em28xx_int_audio_type
which describes the internal audio type (none, ac97, i2s) and is hooked
directly to the device struct. Then get rid of field has_audio in struct
em28xx_audio_mode.

A follow-up patch will then remove struct em28xx_ac97_mode and finally
the whole struct em28xx_audio_mode.

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