hal: fix compilation error after disabling Dolby

Disabling Dolby causes unused variable error during
compilation. Cache adev variable to solve this.

CRs-Fixed: 2046777
Change-Id: I9579d58b7e95cc4ada50fc81b02a29f9604b831e
diff --git a/hal/audio_extn/audio_extn.c b/hal/audio_extn/audio_extn.c
index bc722f8..ad28c44 100644
--- a/hal/audio_extn/audio_extn.c
+++ b/hal/audio_extn/audio_extn.c
@@ -67,6 +67,7 @@
     bool hifi_audio_enabled;
     bool ras_enabled;
     struct aptx_dec_bt_addr addr;
+    struct audio_device *adev;
 };
 
 static struct audio_extn_module aextnmod;
@@ -805,6 +806,7 @@
     aextnmod.addr.nap = 0;
     aextnmod.addr.uap = 0;
     aextnmod.addr.lap = 0;
+    aextnmod.adev = adev;
 
     audio_extn_dolby_set_license(adev);
     audio_extn_aptx_dec_set_license(adev);