ALSA: hda: Drop export of snd_hdac_bus_add/remove_device()

snd_hdac_bus_add_device() and snd_hdac_remove_device() are called only
internally in hda-core.  Let's drop the exports of them and move the
declarations into local.h.

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