ptp clock: Increase tolerance for late follow-up and delay-resp

ptp clock: Increase tolerance for late follow-up and delay-resp

The follow-up and delay-resp messages carry precise
timestamps for the arrival at the clock master, but
the local return time is unimportant, so we should be very
lenient in accepting them late. Some PTP masters don't
prioritise sending those packets, and we reject all the
responses and never sync - or take forever to do so.

Increase the tolerance to 20x the mean path delay.

Also fix a typo in one debug output that would print
the absolute time of the delay-resp message, not the offset
from the delay-req that it's actually being compared against.