net: fec: ptp: correct the ENET_ATCOR value

The current driver adjust freq formula is:
fe * diff = ppb * pc
Note:
  fe: ENET ref clock frequency in Hz
  diff = inc_corr - inc: difference between default increment and correction increment
  ppb: parts per billion adjustment from base
  pc: correction period (in number of fe clock cycles)

The correction increment will be used after N cycles of regular increments,
not every N cycles (with N being the correction period). For example, set ENET_ATCOR=4,
INC=8, INC_CORR=9, there will be 4 increments of 8 (ENET_ATINC[INC]) , followed by 1
increment of 9 (ENET_ATINC[INC_CORR]).

So, the correct formula is:
	fe * diff = ppb * (pc + 1)

For ENET_ATCOR, a value 0 disables the correction counter and no corrections occur.
So base on the origin formula, set pc = pc > 1 ? pc - 1 : pc.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/fec_ptp.c

@@ -353,6 +353,7 @@ static int fec_ptp_adjfreq(struct ptp_clock_info *ptp, s32 ppb)
 	tmp = readl(fep->hwp + FEC_ATIME_INC) & FEC_T_INC_MASK;
 	tmp |= corr_ns << FEC_T_INC_CORR_OFFSET;
 	writel(tmp, fep->hwp + FEC_ATIME_INC);
+	corr_period = corr_period > 1 ? corr_period - 1 : corr_period;
 	writel(corr_period, fep->hwp + FEC_ATIME_CORR);
 	/* dummy read to update the timer. */
 	timecounter_read(&fep->tc);