ramips: disable badblock shifting for MT7621 NAND

ramips: disable badblock shifting for MT7621 NAND

The MediaTek MT7621 NAND driver currently intransparently shifts NAND
pages when a block is marked as bad. Because of this, offsets for e.g.
caldata and MAC-addresses seem to be off.

This is, howeer, not a task for the mtd NAND driver, as the flash
translation layer is tasked with this.

This patch disables this badblock shifting. This fix was originally
proposed by Jo-Philipp Wich at
https://bugs.openwrt.org/index.php?do=details&task_id=1926

Fixes FS#1926 ("MTD partition offset not correctly mapped when bad
eraseblocks present")
Signed-off-by: David Bauer <mail@david-bauer.net>