Previous one was not working when:
* tR and tF were both inside (0; 4000) and tR < tF
* tR and tF were both inside (4000; 8000) and tF < tR

in those cases it was calculating ttrans around falling edge instead of rising
edge. This commit fixes it.

t24p calibration finds the rising&falling edges in
dev/rxts_calibrator.c:rxts_calibration_update.  It then computes the
"transition" as (falling+rising)/2.  That is, ttrans points 25% past the
dangerous transition.  Now that value arrives in one of the three copies of
ptpd_netif_linearize_rx_timestamp (depending on build).  In this method
ttrans has +-1/4 period added to it to compute trip_lo and trip_hi.  The
intent as described by the comment (and common sense) is to avoid the rising
edge when the phase is within +-1/4 period.  Unfortunately, this code
assumes that ttrans IS the rising edge, when in fact it is the rising
edge+25%. Thus, the code ACTUALLY tests phase within 0-50%.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

call t24p calibration every time PTP is set to Slave mode, or read the value from EEPROM when Master mode

New procedure that does not require PTP traffic, is much faster and doesn't kill the stack.
Requires recent endpoint version (wr-cores commit: 4713a98a).
Todo: modify locking functions in libptpnetif to automatically perform the calibration whenever
PTP enters slave mode.