mtd: Convert list_for_each to entry variant

convert list_for_each() to list_for_each_entry() where
applicable.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1623155699-61935-1-git-send-email-zou_wei@huawei.com