mtd: nand: add __nand_correct_data helper function

Split nand_correct_data() into two part, a pure calculation function
and a wrapper for mtd interface.

The tmio_nand driver can implement its ecc.correct function easily
using this __nand_correct_data helper.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>