UBI: fix NOR erase preparation quirk

In 'nor_erase_prepare()' we want to make sure the UBI headers are
corrupted. But it is possible that one of the headers just contains
all 0xFFs, which is also OK, because UBI will erase it in case of
a power cut.
Signed-off-by: NHolger Brunck <holger.brunck@keymile.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/io.c

@@ -507,11 +507,13 @@ static int nor_erase_prepare(struct ubi_device *ubi, int pnum)
 	 * PEB.
 	 */
 	err1 = ubi_io_read_vid_hdr(ubi, pnum, &vid_hdr, 0);
-	if (err1 == UBI_IO_BAD_HDR_EBADMSG || err1 == UBI_IO_BAD_HDR) {
+	if (err1 == UBI_IO_BAD_HDR_EBADMSG || err1 == UBI_IO_BAD_HDR ||
+	    err1 == UBI_IO_FF) {
 		struct ubi_ec_hdr ec_hdr;
 
 		err1 = ubi_io_read_ec_hdr(ubi, pnum, &ec_hdr, 0);
-		if (err1 == UBI_IO_BAD_HDR_EBADMSG || err1 == UBI_IO_BAD_HDR)
+		if (err1 == UBI_IO_BAD_HDR_EBADMSG || err1 == UBI_IO_BAD_HDR ||
+		    err1 == UBI_IO_FF)
 			/*
 			 * Both VID and EC headers are corrupted, so we can
 			 * safely erase this PEB and not afraid that it will be