hal: avoid redundant device switch during playback.

With music playback in background, a new playback is started, current 
snd_device of this new playback usecase is none at this moment, 
then force_routing flag is set to true, which causes device 
switch even if output snd devices are same.

Fix it by checking current snd_device to avoid redundant device switch.

CRs-Fixed: 2273928 
Change-Id: Ic4592123bf0d1d9c1349b61fea2af56cf16cabf6
diff --git a/hal/msm8974/platform.c b/hal/msm8974/platform.c
index 7c950e5..0c28182 100755
--- a/hal/msm8974/platform.c
+++ b/hal/msm8974/platform.c
@@ -6284,7 +6284,8 @@
         ALOGI("%s: new_snd_devices[%d] is %d", __func__, i, new_snd_devices[i]);
         if ((platform_check_codec_backend_cfg(adev, usecase, new_snd_devices[i],
                                              &backend_cfg)) ||
-             (!platform_check_backends_match(usecase->out_snd_device, snd_device))) {
+            ((usecase->out_snd_device != SND_DEVICE_NONE) &&
+             !platform_check_backends_match(usecase->out_snd_device, snd_device))) {
             ret = platform_set_codec_backend_cfg(adev, new_snd_devices[i],
                                            backend_cfg);
             if (!ret) {