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>