fix(emqx_auth_mnesia_api): fix Dialyzer warnings

apps/emqx_auth_mnesia/src/emqx_auth_mnesia_api.erl

@@ -128,8 +128,7 @@ add_clientid(_Bindings, Params) ->
             Re = do_add_clientid(Params),
             case Re of
                 ok -> return(ok);
-                <<"ok">> -> return(ok);
-                _ -> return({error, format_msg(Re)})
+                {error, Error} -> {error, format_msg(Error)}
             end
     end.
 
@@ -178,8 +177,7 @@ add_username(_Bindings, Params) ->
         false ->
             case do_add_username(Params) of
                 ok -> return(ok);
-                <<"ok">> -> return(ok);
-                Error -> return({error, format_msg(Error)})
+                {error, Error} -> {error, format_msg(Error)}
             end
     end.
 
@@ -310,4 +308,3 @@ urldecode(S) ->
 urldecode(S) ->
     http_uri:decode(S).
 -endif.
-