ALSA: hda: fix the missing ptr initialization

ebus is a member of extended device and was never initialized, so
do this at device creation.

Signed-off-by: Jeeja KP <jeeja.kp@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>