lib/mac: fix raise ValueError

Traceback (most recent call last):
  File "/home/openflow/ryu/.venv/local/lib/python2.7/site-packages/nose/tools/nontrivial.py", line 67, in newfunc
    raise AssertionError(message)
AssertionError: test_mac_haddr_to_bin_true() did not raise ValueError

Signed-off-by: EisukeTAKAHASHI <takahashi.eisuke@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>