hal: Add HFP volume mixer control on MSM8996

MSM8996 target uses Primary AUXPCM for Hands Free
Profile(HFP). Update volume mixer control for HPF.

Change-Id: I412f5e6a71246d9002f002584c7fde07d35ee604
diff --git a/hal/audio_extn/hfp.c b/hal/audio_extn/hfp.c
index a73dfa1..3e09e55 100644
--- a/hal/audio_extn/hfp.c
+++ b/hal/audio_extn/hfp.c
@@ -47,6 +47,8 @@
 
 #ifdef PLATFORM_MSM8994
 #define HFP_RX_VOLUME     "SEC AUXPCM LOOPBACK Volume"
+#elif defined PLATFORM_MSM8996
+#define HFP_RX_VOLUME     "PRI AUXPCM LOOPBACK Volume"
 #else
 #define HFP_RX_VOLUME     "Internal HFP RX Volume"
 #endif