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