iwlwifi: mvm: combine parts of UMAC and LMAC sched scans

Similarly to the regular scan patch, a lot of the UMAC and LMAC sched
scan code is also 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>