UBI: fix bug in image sequence number handling

This patch fixes a bug in the image seq. number handling in the
scanning level. The assignment of the image_seq was incorrect.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>