UBI: simplify image sequence test

The test:
if (!a && b)
  a = b;
can be symplified in:
if (!a)
  a = b;

And there's no need to test if ubi->image_seq is not null, because if it is,
it is set to image_seq.
So, we just test if image_seq is not null.
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/attach.c

@@ -900,10 +900,9 @@ static int scan_peb(struct ubi_device *ubi, struct ubi_attach_info *ai,
 		 * number.
 		 */
 		image_seq = be32_to_cpu(ech->image_seq);
-		if (!ubi->image_seq && image_seq)
+		if (!ubi->image_seq)
 			ubi->image_seq = image_seq;
-		if (ubi->image_seq && image_seq &&
-		    ubi->image_seq != image_seq) {
+		if (image_seq && ubi->image_seq != image_seq) {
 			ubi_err("bad image sequence number %d in PEB %d, expected %d",
 				image_seq, pnum, ubi->image_seq);
 			ubi_dump_ec_hdr(ech);