mtd: rfd_ftl: fix use-after-free

del_mtd_blktrans_dev() will kfree part, so after this call both part and
dev point to freed memory. Move the call to avoid use-after-free.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210807214538.14484-5-sean@mess.org