e1000: add smart power down code

Smart Power Down is a power saving feature in newer e1000 hardware. We
disable it because it causes time to link to be long, but make it a
user choice.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>

diff --git a/drivers/net/e1000/e1000.h b/drivers/net/e1000/e1000.h
index 2b96ad0..dbdaa33 100644
--- a/drivers/net/e1000/e1000.h
+++ b/drivers/net/e1000/e1000.h
@@ -338,6 +338,7 @@
 #ifdef NETIF_F_TSO
 	boolean_t tso_force;
 #endif
+	boolean_t smart_power_down;	/* phy smart power down */
 	unsigned long flags;
 };