mtd: fix repeated word in comment

Delete the redundant word 'in'.

Signed-off-by: wangjianli <wangjianli@cdjrlc.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220908122229.10814-1-wangjianli@cdjrlc.com