net: fec: ptp: Use the 31-bit ptp timer.

When ptp switches from software adjustment to hardware ajustment, linux ptp can't converge.
It is caused by the IP limit. Hardware adjustment logcial have issue when ptp counter
runs over 0x80000000(31 bit counter). The internal IP reference manual already remove 32bit
free-running count support. This patch replace the 32-bit PTP timer with 31-bit.

Signed-off-by: Luwei Zhou <b45643@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>