net: dp83640: fix broken calibration routine.

Currently, the calibration function that corrects the initial offsets
among multiple devices only works the first time.  If the function is
called more than once, the calibration fails and bogus offsets will be
programmed into the devices.

In a well hidden spot, the device documentation tells that trigger indexes
0 and 1 are special in allowing the TRIG_IF_LATE flag to actually work.

This patch fixes the issue by using one of the special triggers during the
recalibration method.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>