iwlwifi: mvm: combine parts of UMAC and LMAC scans

A lot of the UMAC and LMAC scan code is almost identical.  Grab the
low hanging fruits by combining the obvious parts.

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