mtd: rawnand: omap: Fix kernel doc warning on 'calcuate' typo

Fix a trivial typo which is reported after enabling W=1 level of warnings:

  drivers/mtd/nand/raw/omap2.c:927: warning: expecting prototype
    for omap_calcuate_ecc(). Prototype was for omap_calculate_ecc() instead

Signed-off-by: Vladimir Molokov <vladimir@molokov.se>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210801205909.7102-1-vladimir@molokov.se