mtd: add ecc_strength fields to mtd structs

This adds 'ecc_strength' to struct mtd_info.  This stores the maximum number of
bit errors that can be corrected in one writesize region.

For consistency with the nand code, 'strength' is similiarly added to struct
nand_ecc_ctrl.  This stores the maximum number of bit errors that can be
corrected in one ecc step.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>