mtd: nand: atmel: correct bitflips in erased pages for pre-sama5d4 SoCs

New atmel SoCs are able to fix bitflips in erased pages, but old ones
are still impacted by this problem. Use nand_check_erased_ecc_chunk() to
handle this case.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Herve Codina <herve.CODINA@celad.com>
Reviewed-by: Herve Codina <herve.CODINA@celad.com>
Tested-by: Herve Codina <herve.CODINA@celad.com>