hal: Enable parsing of interface tag independent of backend_tag

- Current platform_info file parsing assumes backend tag to be present
always with hw_interface tag.
-With multiCopp changes as we have to have hw_interface entry for
all supported device, it should not be mandatory to have backend_tag.
-Decouple hw_interface from backend_tag.

Change-Id: I1c1163273d92305deac787ccecdcceb1f6943532
diff --git a/hal/msm8916/platform.c b/hal/msm8916/platform.c
index fad2c04..0870bde 100644
--- a/hal/msm8916/platform.c
+++ b/hal/msm8916/platform.c
@@ -5323,10 +5323,13 @@
           platform_get_snd_device_name(device),
           backend_tag_table[device] != NULL ? backend_tag_table[device]: "null",
           backend_tag);
-    if (backend_tag_table[device]) {
-        free(backend_tag_table[device]);
+
+    if (backend_tag != NULL ) {
+        if (backend_tag_table[device]) {
+           free(backend_tag_table[device]);
+        }
+        backend_tag_table[device] = strdup(backend_tag);
     }
-    backend_tag_table[device] = strdup(backend_tag);
 
     if (hw_interface != NULL) {
         if (hw_interface_table[device])