ALSA: hdac: Remove usage of struct hdac_ext_bus and use hdac_bus instead

This patch removes the hdac_ext_bus structure. The legacy and
enhanced HDaudio capabilities can be handled in a backward-compatible
way without separate definitions.

Follow-up patches in this series handle the driver definition.

Signed-off-by: Rakesh Ughreja <rakesh.a.ughreja@intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>