UBI: Init vol->reserved_pebs by assignment

`vol' is a newly allocated value by kzalloc. Initialize it by assignment
instead of `+='.
Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/vmt.c

@@ -253,7 +253,7 @@ int ubi_create_volume(struct ubi_device *ubi, struct ubi_mkvol_req *req)
 
 	/* Calculate how many eraseblocks are requested */
 	vol->usable_leb_size = ubi->leb_size - ubi->leb_size % req->alignment;
-	vol->reserved_pebs += div_u64(req->bytes + vol->usable_leb_size - 1,
+	vol->reserved_pebs = div_u64(req->bytes + vol->usable_leb_size - 1,
 				     vol->usable_leb_size);
 
 	/* Reserve physical eraseblocks */