mtd: fix mtd_oobavail() incoherent returned value

mtd_oobavail() returns either mtd->oovabail or mtd->oobsize. Both
values are unsigned 32-bit entities, so there is no reason to pretend
returning a signed one.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>