mtd: nand: omap: Fix comment in platform data using wrong Kconfig symbol

The symbol that is being used in the #if/#endif block is not the one
which is mentioned at the bottom.

Fixes: 93af53b8

 ("nand: omap2: Remove horrible ifdefs to fix module probe")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>