mtd: atmel_nand: use minimum ecc requirements of nand: ecc_{strength,step}_ds

Since ecc_{strength,step}_ds is introduced in nand_chip structure for
minimum ecc requirements. So we can use them directly and remove our
own get_onfi_ecc_param function.

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