mtd: atmel_nand: remove pmecc_bytes_per_sector, use chip->ecc.bytes instead

For PMECC, the pmecc_bytes_per_sector has same meaning as ecc.bytes.
So remove pmecc_bytes_per_sector and use ecc.bytes instead.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>