a2dp: Fix BT SCO Tx configuration during a2dp to sco transition

 -For voice call tx backend is configured with a2dp sampling rate
  leading to distorted audio during voice call.
 -Don't set tx backend sample rate for a2dp if abr is not enabled.

Change-Id: I7cdb0a5c98b550665cfaafb02cd9151579d7138c
diff --git a/hal/audio_extn/a2dp.c b/hal/audio_extn/a2dp.c
index 19c839c..36065c9 100644
--- a/hal/audio_extn/a2dp.c
+++ b/hal/audio_extn/a2dp.c
@@ -1183,20 +1183,21 @@
                     rate_str = SPEECH_TX_SAMPLE_RATE;
                 else
                     rate_str = ABR_TX_SAMPLE_RATE;
-            }
-            ALOGD("%s: set backend tx sample rate = %s", __func__, rate_str);
-            ctl_sample_rate = mixer_get_ctl_by_name(a2dp.adev->mixer,
-                                            MIXER_SOURCE_SAMPLE_RATE_TX);
-            if (!ctl_sample_rate) {
-                ALOGE("%s: ERROR backend sample rate mixer control not identifed", __func__);
-                is_configured = false;
-                goto fail;
-            }
 
-            if (mixer_ctl_set_enum_by_string(ctl_sample_rate, rate_str) != 0) {
-                ALOGE("%s: Failed to set backend sample rate = %s", __func__, rate_str);
-                is_configured = false;
-                goto fail;
+                ALOGD("%s: set backend tx sample rate = %s", __func__, rate_str);
+                ctl_sample_rate = mixer_get_ctl_by_name(a2dp.adev->mixer,
+                                                MIXER_SOURCE_SAMPLE_RATE_TX);
+                if (!ctl_sample_rate) {
+                    ALOGE("%s: ERROR backend sample rate mixer control not identifed", __func__);
+                    is_configured = false;
+                    goto fail;
+                }
+
+                if (mixer_ctl_set_enum_by_string(ctl_sample_rate, rate_str) != 0) {
+                    ALOGE("%s: Failed to set backend sample rate = %s", __func__, rate_str);
+                    is_configured = false;
+                    goto fail;
+                }
             }
         }
     } else {