cfm: No longer trigger fault upon unexpected ccm_interval.

cfm: No longer trigger fault upon unexpected ccm_interval.

According to the 802.1ag specification, when a CCM is received
which advertises a misconfigured transmission interval, a fault
should be triggered.  This patch goes against the spec by simply
warning when this happens.  This is done for several reasons.

  - Faults can cause controllers to make potentially expensive
    changes in the network topology.
  - Faults can be maliciously triggered by crafting invalid CCMs.
  - Reducing the number of places in the code where rmp->fault and
    cfm->fault are changed makes the code easier to debug and
    reason about.