When offset from master is more than one second, the code used to
time->get, subtract and then time->set.

However, in white rabbit we cannot get the WR time and time->get
returns the unix time instead. The only way to have hardware time is
timestamping a frame.  Thus, use the master's "T4" that we just
received plus the one-way-delay as an approximation of the current
hardware time. Later T1..T4 tuples will complete the fine
synchronization.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>