[finsh] fix compiling warning.

components/finsh/msh_cmd.c

@@ -243,6 +243,11 @@ int cmd_mkfs(int argc, char** argv)
         return 0;
     }
     
+    if (result != RT_EOK)
+    {
+        rt_kprintf("mkfs failed, result=%d\n", result);
+    }
+
     return 0;
 }
 FINSH_FUNCTION_EXPORT_ALIAS(cmd_mkfs, __cmd_mkfs, format disk with file system);