[ALSA] AC97 bus interface for ad-hoc drivers

AC97 Codec,PCI drivers
I've made the review changes and as requested I've pasted the RFC by
Nicolas below:-

'I would like to know what people think of the following patch.  It
allows for a codec on an AC97 bus to be shared with other drivers which
are completely unrelated to audio.  It registers a new bus type, and
whenever a codec instance is created then a device for it is also
registered with the driver model using that bus type.  This allows, for
example, to use the extra features of the UCB1400 like the touchscreen
interface and the additional GPIOs and ADCs available on that chip for
battery monitoring.  I have a working UCB1400 touchscreen driver here
that simply registers with the driver model happily working alongside
with audio features using this.'

Changes over RFC:-

  o Now matches codec name within codec group.
  o Added ac97_dev_release() to stop kernel complaining about no release
method for device.
  o Added 'config SND_AC97_BUS' to sound/pci/Kconfig and moved 'config
SND_AC97_CODEC' out with the PCI=n statement.
  o module is now called snd-ac97-bus

Signed-off-by: Liam Girdwood <liam.girdwood@wolfsonmicro.com>
Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>