mip: Fix Rev Revocation [Ack] Flags dissection bugs.

  Backported manually from master (ga3510cc).

Change-Id: I1b92f488cffaa51cc00067b3dd353975702ad911
Reviewed-on: https://code.wireshark.org/review/5836
Reviewed-by: Bill Meier <wmeier@newsguy.com>