pmc_agent: Use longer update interval when not subscribed.

When phc2sys is started with the -w option, the pmc agent is not
subscribed to events by the pmc_agent_subscribe() function, which also
sets the update interval. The update interval in this case is zero,
which means the pmc agent is trying to update the currentUtcOffset value
on every call of pmc_agent_update(), i.e. on every clock update in
phc2sys.

Set a default update interval of 60 seconds to reduce the rate of
pmc requests.

Fixes: e3ca7ea90a9e ("pmc_agent: Make update interval configurable.")
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>