hf mf sim: Multiple fixes from review of PR #209.

hf mf sim: Multiple fixes from review of PR #209.

- Don't increment the nonce when random mode is disabled (this breaks the
  standard attack).

- Don't attempt the standard attack when random mode is enabled (there's no
  point as it won't work, per comments from @pwpiwi).

- Attempt the moebius attack if the standard attack fails.