If hwPimSGDrRegisterState is 'prune', this is the time
remaining before the local router sends a Null-Register
message to the RP. If hwPimSGDrRegisterState is
'joinPending', this is the time remaining before the local
router resumes encapsulating data packets and sending them
to the RP. Otherwise, this is zero. This timer is called
the Register-Stop Timer in the PIM-SM specification.