iwlwifi: mvm: spin off a function to start scan offload

The net-detect feature will require a scan offload to be started in
the same way it is done now for scheduled scan.  Spin a new function
off of the sched_scan_start op code for reuse.  Additionally,
restructure the function a bit for more readability.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>