audio: fix echo reference channels configuration

When an auxiliary mic channel is used, the echo reference
should use only the main channels to be consistent with the
way the reverse effect processing is configured.

Change-Id: I28ee1e2a9852fdd0e904fb01bedf90f3372683c9
diff --git a/audio/audio_hw.c b/audio/audio_hw.c
index f2fda76..2caba05 100644
--- a/audio/audio_hw.c
+++ b/audio/audio_hw.c
@@ -2202,7 +2202,7 @@
     if (in->need_echo_reference && in->echo_reference == NULL)
         in->echo_reference = get_echo_reference(adev,
-                                        in->config.channels,
+                                        popcount(in->main_channels),
     /* this assumes routing is done previously */