ubifs: Fix ubifs_assert_cmt_locked()

U-Boot does not implement down_write_trylock() and its stub always returns
true that lock was acquired. Therefore ubifs_assert_cmt_locked() assert
currently always fails.

Fix this issue by redefining ubifs_assert_cmt_locked() to just empty stub
as there is nothing to assert.

Signed-off-by: Pali Rohár <pali@kernel.org>