mtd: rawnand: bcm47xx: fix spelling typo in comment

Fix spelling typo in comment.

Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220905063232.1830197-1-13667453960@163.com