The ecc layout we provide to the nand core is broken since
00f119a293. Before this commit ecc bytes/size were per page
and now they are per subpage. The offset for the first ecc
byte was calculated as oob_end - num_ecc_bytes which must
now become oob_end - num_ecc_bytes * ecc_steps.

Fixes: 00f119a293 ("mtd: omap gpmc: fix ecc bytes/size")

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>