atmel_nand: use nand ecc_{strength, step}_ds instead of our own function

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>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>