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: NMiquel Raynal <miquel.raynal@bootlin.com>

include/linux/mtd/mtd.h

@@ -392,7 +392,7 @@ static inline void mtd_set_ooblayout(struct mtd_info *mtd,
 	mtd->ooblayout = ooblayout;
 }
 
-static inline int mtd_oobavail(struct mtd_info *mtd, struct mtd_oob_ops *ops)
+static inline u32 mtd_oobavail(struct mtd_info *mtd, struct mtd_oob_ops *ops)
 {
 	return ops->mode == MTD_OPS_AUTO_OOB ? mtd->oobavail : mtd->oobsize;
 }