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>
diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 60ad28e..ad61406 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -1031,6 +1031,18 @@
 }
 EXPORT_SYMBOL_GPL(mtd_read_oob);
 
+int mtd_write_oob(struct mtd_info *mtd, loff_t to,
+				struct mtd_oob_ops *ops)
+{
+	ops->retlen = ops->oobretlen = 0;
+	if (!mtd->_write_oob)
+		return -EOPNOTSUPP;
+	if (!(mtd->flags & MTD_WRITEABLE))
+		return -EROFS;
+	return mtd->_write_oob(mtd, to, ops);
+}
+EXPORT_SYMBOL_GPL(mtd_write_oob);
+
 /**
  * mtd_ooblayout_ecc - Get the OOB region definition of a specific ECC section
  * @mtd: MTD device structure