mtd: rawnand: omap: Aggregate the HW configuration of the ELM

Instead of calling elm_config() for each possible BCH configuration,
just save the BCH configuration that must be applied and use it in a
single call at the bottom.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210610134906.3503303-2-miquel.raynal@bootlin.com