A few basic codes for communicating over HD-audio bus are moved to
struct hdac_bus now.  It has only command and get_response ops in
addition to the unsolicited event handling.

Note that the codec-side tracing support is disabled temporarily
during this transition due to the code shuffling.  It will be
re-enabled later once when all pieces are settled down.

Signed-off-by: Takashi Iwai <tiwai@suse.de>