fs: ext4: use fs_ls_generic

Now that opendir, readir, closedir are implemented for ext4 we can use
fs_ls_generic() for implementing the ls command.

Adjust the unit tests:

* fs_ls_generic() produces more spaces between file size and name.
* The ext4 specific message "** Can not find directory. **\n" is not
  written anymore.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/py/tests/test_fs/test_basic.py

@@ -33,10 +33,7 @@ class TestFsBasic(object):
             # In addition, test with a nonexistent directory to see if we crash.
             output = u_boot_console.run_command(
                 '%sls host 0:0 invalid_d' % fs_type)
-            if fs_type == 'ext4':
-                assert('Can not find directory' in output)
-            else:
-                assert('' == output)
+            assert('' == output)
 
     def test_fs2(self, u_boot_console, fs_obj_basic):
         """