WPC with GM configuration: DPLL phase offset is not updated in holdover

    DPLL phase offset is stuck on the last value when DPLL enters holdover. 

latest master    

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    1.Setup grandmaster with two-NIC configuration
    2. On the NIC without GNSS module change SMA1 pin direction, so incoming PPS signal stops
    3. The DPLL enters holdover. 

The reported DPLL phase offset metric remains stuck on the last value     

    The reported DPLL phase offset metric jumps to the maximum value