mtd: remove unneeded break

A break is not needed if it is preceded by a return

Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201019191631.9604-1-trix@redhat.com