mtd: Uninline mtd_write_oob and move it to mtdcore.c

There's no reason for having mtd_write_oob inlined in mtd.h header.
Move it to mtdcore.c where it belongs.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>