ALSA: hda - introduce HDA_SUBDEV_AMP_FLAG (ControlAmp in proc)

The purpose of this changeset is to show information about amplifier
setting in the codec proc file. Something like:

  Control: name="Front Playback Volume", index=0, device=0
    ControlAmp: chs=3, dir=Out, idx=0, ofs=0
  Control: name="Front Playback Switch", index=0, device=0
    ControlAmp: chs=3, dir=In, idx=2, ofs=0

Signed-off-by: Jaroslav Kysela <perex@perex.cz>