The count of cable modems that are registered and online
on this cable mac interface. Registered cable modems are
those with one of the following values.
registrationComplete(6) of docsIfCmtsCmStatusValue OR
either of online(12), kekRejected(10),
onlineKekAssigned(6), tekRejected(11), onlineTekAssigned(7)
of cdxCmtsCmStatusValue