igb: update comments for serdes config and update to handle duplex

This update corrects the driver so that it handles duplex for serdes links
correctly instead of just forcing full duplex always.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/igb/e1000_hw.h b/drivers/net/igb/e1000_hw.h
index 119869b..b1e0c06 100644
--- a/drivers/net/igb/e1000_hw.h
+++ b/drivers/net/igb/e1000_hw.h
@@ -50,6 +50,7 @@
 #define E1000_REVISION_2 2
 #define E1000_REVISION_4 4
 
+#define E1000_FUNC_0     0
 #define E1000_FUNC_1     1
 
 enum e1000_mac_type {