HAL: Add new device for voice headset

3-pole headphones not working for voice
usecase due to different backend added
for headset.

add new device for headset to differentiate
from headphones and update mixer path
accordingly.

Change-Id: I29d58e29637534e7d2b6098c5cb89ca6f3b987db
diff --git a/hal/msm8974/platform.h b/hal/msm8974/platform.h
index c23ea6d..fba1077 100644
--- a/hal/msm8974/platform.h
+++ b/hal/msm8974/platform.h
@@ -106,6 +106,7 @@
     SND_DEVICE_OUT_VOICE_SPEAKER_2,
     SND_DEVICE_OUT_VOICE_SPEAKER_2_VBAT,
     SND_DEVICE_OUT_VOICE_HEADPHONES,
+    SND_DEVICE_OUT_VOICE_HEADSET,
     SND_DEVICE_OUT_VOICE_LINE,
     SND_DEVICE_OUT_HDMI,
     SND_DEVICE_OUT_SPEAKER_AND_HDMI,