e1000e: add comment to spinlock_t definition

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/e1000.h

@@ -372,7 +372,7 @@ struct e1000_adapter {
 	/* structs defined in e1000_hw.h */
 	struct e1000_hw hw;
 
-	spinlock_t stats64_lock;
+	spinlock_t stats64_lock;	/* protects statistics counters */
 	struct e1000_hw_stats stats;
 	struct e1000_phy_info phy_info;
 	struct e1000_phy_stats phy_stats;