Define the common hd-audio driver and device types to bind over
snd_hda_bus_type publicly.  This allows to implement other type of
device and driver code over hd-audio bus.

Now both struct hda_codec and struct hda_codec_driver inherit these
new struct hdac_device and struct hdac_driver, respectively.

The bus registration is done in subsys_initcall() to assure it
before any other driver registrations.
Signed-off-by: Takashi Iwai <tiwai@suse.de>