audio: hal: enable ext FM usecase via audio patch

Support external FM Tuner usecase via device
to device audio patch configuration.

Change-Id: I876ca3f1b4aeb79593a75128ff640083803e07fe
diff --git a/hal/msm8974/platform.c b/hal/msm8974/platform.c
index 0321714..2c1fef1 100644
--- a/hal/msm8974/platform.c
+++ b/hal/msm8974/platform.c
@@ -480,7 +480,7 @@
                                              NAV_GUIDANCE_PCM_DEVICE},
     [USECASE_AUDIO_PLAYBACK_PHONE] = {PHONE_PCM_DEVICE,
                                       PHONE_PCM_DEVICE},
-
+    [USECASE_AUDIO_FM_TUNER_EXT] = {-1, -1},
 };
 
 /* Array to store sound devices */