ENGR00288342 net:fec_ptp: fix the potential issue for storing timestamp

ENGR00288342 net:fec_ptp: fix the potential issue for storing timestamp

The timestamps generated in the i.MX drivers are generated by the
nanoseconds part coming from the 1588 clock. But the number of seconds
are maintained in a private structure of the interface. Those are
updated in a 1588 clock rollover interrupt.

The timestamp is generated right before a rollover of a second and the
timestamp value is constructed afterwards. Therefore the bigger part of
the timestamp is wrong (the second).

Suggested solution (pseudo-code):
If( actual-time.nsec < timestamp.nsec )
	Timestamp.sec = fpp->prtc -1;
Else
	Timestamp.sec = fpp->prtc;

Signed-off-by: Fugang Duan <B38611@freescale.com>