hal: avoid setting stereo channel mask to voip usecase

Voip usecase is started with stereo channel mask. This is
not allowed due to voip_rx profile is not supported.

Fix it by setting channel mask to mono for voip usecase.

CRs-Fixed: 2418048
Change-Id: I949808076a56cf2e43f0309ce1656646e0495087
diff --git a/hal/audio_hw.c b/hal/audio_hw.c
index cd8f22e..6399cb2 100644
--- a/hal/audio_hw.c
+++ b/hal/audio_hw.c
@@ -6463,8 +6463,7 @@
         if (!voice_extn_is_compress_voip_supported()) {
             if (out->sample_rate == 8000 || out->sample_rate == 16000 ||
              out->sample_rate == 32000 || out->sample_rate == 48000) {
-                //FIXME: add support for MONO stream configuration when audioflinger mixer supports it
-                out->channel_mask = AUDIO_CHANNEL_OUT_STEREO;
+                out->channel_mask = AUDIO_CHANNEL_OUT_MONO;
                 out->usecase = USECASE_AUDIO_PLAYBACK_VOIP;
                 out->format = AUDIO_FORMAT_PCM_16_BIT;