mtd: nand: Base BCH ECC bytes on required strength

NAND devices with page sizes over 4 KiB require more than 4-bits of ECC
coverage. This patch calculates the value of ecc_bytes based on a still
assumed 512-byte step size (13-bits) and the ecc_strength.

Example:
Micron M73A devices (8 KiB page) require 8-bit ECC per 512-byte

Signed-off-by: Jordan Friendshuh <jfriendshuh@xes-inc.com>
Signed-off-by: Aaron Sierra <asierra@xes-inc.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>