mtd: nand: fix typo in comment

Funny one :) "Heck" fits somehow, too, but I am sure it was meant to be "Check".

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>