UBI: Fix build problem noticed on Apollon (arm/testing repo)

This patch fixes a build problem noticed on Apollon by using
mtd_dev_by_eb() instead of "/" as done in the Linux UBI version.
So this brings the U-Boot UBI version more in sync with the Linux
version again.

Signed-off-by: Stefan Roese <sr@denx.de>

drivers/mtd/ubi/build.c

@@ -536,7 +536,7 @@ static int io_init(struct ubi_device *ubi)
 	 */
 
 	ubi->peb_size   = ubi->mtd->erasesize;
-	ubi->peb_count  = ubi->mtd->size / ubi->mtd->erasesize;
+	ubi->peb_count  = mtd_div_by_eb(ubi->mtd->size, ubi->mtd);
 	ubi->flash_size = ubi->mtd->size;
 
 	if (ubi->mtd->block_isbad && ubi->mtd->block_markbad)