iwlwifi: mvm: always use iwl_mvm_scan_size to calculate the scan size

We have a function (iwl_mvm_scan_size()) that can calculate the scan
size for both UMAC and LMAC scans.  Use that function instead of
calculating manually for LMAC scan and sched scan.

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>