ALSA: hdac: Remove the usage of key for host stream

hdac_ext_stream assign doesn't require key mapping as in case of
hdac_stream. So for host stream, the key to device mapping needs
to be removed.

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>