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>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index cd1f557..e3549f0 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -392,7 +392,7 @@
 	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;
 }