mtd: atmel_nand: remove pmecc_sector_number, use ecc.steps instead

For PMECC, the pmecc_sector_number has same meaning as ecc.steps.
So use ecc.steps to replace the pmecc_sector_number.

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