mixer_paths: enable loopback port for TTY HCO

Missed usecase in previous commit

Bug: 19240678

Change-Id: I5ccbcc787a109efd19924f67abb0e38a3bae3d92
diff --git a/mixer_paths.xml b/mixer_paths.xml
index 408f268..c2439f2 100644
--- a/mixer_paths.xml
+++ b/mixer_paths.xml
@@ -975,6 +975,8 @@
 
 	<path name="voice-tty-hco-handset">
 		<ctl name="TTY Mode" value="HCO" />
+	    <!-- Port mixer is common bit for all voice verbs -->
+		<ctl name="QUAT_MI2S_RX Port Mixer SLIM_1_TX" value="1" />
 		<path name="voice-handset" />
 	</path>