mtd: nand: fix 512 byte software ecc support

Type of 'byte_addr' needes to be 'unsigned int' for 512 byte
ECC support.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>