Like for any other compilation option, use the IS_ENABLED() macro
instead of hardcoding it.

By droping this helper we can get rid of the BCH header in nandsim.c.
Signed-off-by: NMiquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200929230124.31491-7-miquel.raynal@bootlin.com