mtd: atmel_nand: pmecc: fix failure to correct bit error in 1024-bytes sector

The PMECC use BCH algorithm to correct error. In BCH algorithm, the primitive
polynomial value is GF(2^13) for 512-bytes sector size. And it is GF(2^14) for
1024-bytes sector size.

This patch will choose correct degree of the remainders (13 or 14) for
different sector size.

Tested in AT91SAM9X5-EK with MLC nand flash.

More detail can be found in §5.4.1 of:
  AT91SAM ARM-based Embedded MPU Application Note
  <http://www.atmel.com/Images/doc11127.pdf

>

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>