mtd: nand: use the nand_read_subpage index everywhere

Now that the index variable is correctly set earlier in this function
we can use it in other places that compute the same thing too.

Signed-off-by: Ron Lee <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>

drivers/mtd/nand/nand_base.c

···
 	 * ecc.pos. Let's make sure that there are no gaps in ECC positions.
 	 */
 	for (i = 0; i < eccfrag_len - 1; i++) {
-		if (eccpos[i + start_step * chip->ecc.bytes] + 1 !=
-			eccpos[i + start_step * chip->ecc.bytes + 1]) {
+		if (eccpos[i + index] + 1 != eccpos[i + index + 1]) {
 			gaps = 1;
 			break;
 		}