mtd: rawnand: omap2: Fix number of bitflips reporting with ELM

omap_elm_correct_data() returns the number of bitflips for the whole
page. This is wrong, it should return the maximum number of bitflips
found in each ECC step.

In my case with a 4k page size NAND mtcdore reported -EUCLEAN with
only 12 bitflips on a page where we could correct up to 128 bits per
page (provided they are distributed equally on the 8 ECC steps)

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>