The value of tmnxPtpClockClass indicates the local clock class. The
value of this object corresponds to the defaultDS.clockQuality data
set member defined in IEEE Std 1588-2008.
When tmnxPtpClockClockType is 'ordinarySlave (1)',
tmnxPtpClockClass is 'slaveOnly (255)'. Otherwise, when
tmnxPtpClockProfile is 'ieee1588-2008 (2)' tmnxPtpClockClass is
'default (248)'. When tmnxPtpClockProfile is 'g8265dot1-2010 (1)',
tmnxPtpClockClass changes depending on the value of
tmnxSyncIfTimingSystemQltyLevel.
The mapping between ITU-T synchronization quality levels and PTP clock
class is given in ITU-T recommendation G.8265.1.