colibri-imx6ull: fix nand bch geometry

Fix NAND BCH geometry as otherwise the following errors are observed
upon boot:

...
Loading Environment from NAND... NAND read from offset 380000 failed -74
...
NAND read from offset 800 failed -74
...
ubi0 error: ubi_io_read: error -74 (ECC error) while reading 64 bytes
 from PEB 0:0, read 64 bytes
...

Fixes: ed48490f
("mtd: gpmi: fix the bch setting backward compatible issue")
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>