bonding: comparing a u8 with -1 is always false

slave->duplex is a u8 type so the in bond_info_show_slave() when we
check "if (slave->duplex == -1)", it's always false.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
index 45f00b6..de33de1 100644
--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h
@@ -1097,10 +1097,12 @@
 #define SPEED_1000		1000
 #define SPEED_2500		2500
 #define SPEED_10000		10000
+#define SPEED_UNKNOWN		-1
 
 /* Duplex, half or full. */
 #define DUPLEX_HALF		0x00
 #define DUPLEX_FULL		0x01
+#define DUPLEX_UNKNOWN		0xff
 
 /* Which connector port. */
 #define PORT_TP			0x00