iwlwifi: mvm: combine UMAC scans into one

With just a few differences left in the UMAC scan functions now, we
can merge them into one, taking care of the small difference according
to the total number of iterations required.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>