ALSA: hdac: Add API for removing hdac extended device

The HDAC extended device objects are created by HDAC extended bus on probe.
When controller is removed they should be removed as well, so add API
snd_hdac_ext_bus_device_remove for this

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