UBI: rename UBI_SCAN_UNKNOWN_EC

Rename the constant to UBI_UNKNOWN, for the same reason that we are going
to add nother attaching method and re-use the same data structures, so the
"SCAN" in the name becomes incorrect. I've also removed the "_EC" part because
Joel is going to use this constant for other fields in the attaching info data
structures.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>