iwlwifi: mvm: Alter passive scan fragmentation parameters in case of multi-MAC

Make passive scan fragmentation depends on the number of active
interfaces. In case of single-MAC, make passive scan less fragmented.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Reviewed-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>