iwlwifi: mvm: refactor UMAC scan UID handling

We can only have one scan of each type running at the same time, so we
can remove one attribute in the UID information we save.  We had array
index, UID and type, but only UID (== array_index) and type are
necessary.  Refactor the code to use this simplified array.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>