mtd: nand: use a local variable to simplify the nand_scan_tail

There are too many "chip->ecc" in the nand_scan_tail() which makes the eyes
sore.

This patch uses a local variable "ecc" to replace the "chip->ecc" to
make the code more graceful.

Do the code change with "s/chip->ecc\./ecc->/g" in the nand_scan_tail,
and also change some lines by hand.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>