ALSA: hdac: Add snd_hdac_get_hdac_stream()

Add a helper to find the stream using stream tag and direction.
This is useful for drivers to query stream based on stream tag
and direction, fox example while downloading FW thru DSP loader
code

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>