iwlwifi: mvm: introduce a new fragmented scan type: fast balance

Fast balance scan is similar to SCAN_TYPE_MILD, but this scan is
fragmented and has shorter out of operating channel time,
and therefore better match low latency scenarios.

Signed-off-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>