dracut-functions: get_persistent_dev() fall back to the original name

In case of raw disk/partition, ex. /dev/vda1, which doesn't contain any
filesystem on it. get_persistent_dev() would return empty. Now fix it to
return its original name, /dev/vda1 in above case. So that we don't have
to check its return string every time.

Signed-off-by: WANG Chao <chaowang@redhat.com>

dracut-functions.sh

@@ -388,6 +388,7 @@ get_persistent_dev() {
             return
         fi
     done
+    echo $1
 }
 
 expand_persistent_dev() {