wifi: iwlwifi: mvm: Refactor scan start

To support staring internal scan, refactor the scan code such
that the body iwl_mvm_reg_scan_start() is now moved to a local
function that can be used by other flows as well.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Reviewed-by: Ayala Beker <ayala.beker@intel.com>
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Link: https://msgid.link/20240311081938.e7833d4cfc53.I43c1c533c2a5243229002fde6360d423946c54fa@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>