mtd: Add __nand_calculate_ecc() to NAND ECC functions

Add __nand_calculate_ecc() which does not take struct mtd_info.
The built-in 256/512 software ECC calculation and correction tester
will use it.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>