iwlwifi: scan: don't pass large argument by value

Function iwl_mvm_scan_set_legacy_probe_req() second argument
size is too large to be passed by value.
Fix it to be passed by reference.

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