mtd: devices: Remove superfluous "break"

Remove superfluous "break", as there is a "return" before it.

Signed-off-by: Ding Senjie <dingsenjie@yulong.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210514124451.20352-1-dingsenjie@163.com