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>