mtd: sh_flctl: use devm_* managed allocators

This simplifies error and cleanup code paths.

Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>