i40iw: pass hw_stats by reference rather than by value

passing hw_stats by value requires a 280 byte copy so instead
pass it by reference is much more efficient.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Chien Tin Tung <chien.tin.tung@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>