iwlwifi: mvm: combine UMAC and LMAC scan_stop functions

The UMAC and LMAC scan_stop functions are now nearly identical, so
they can be combined into a single function instead.

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>