[SUNGEM]: Add tx_lock

Using tx_lock in SunGEM makes the driver logic in conformance
with tg3 and e1000, easing maintainance.
Signed-off-by: Eric Lemoine <eric.lemoine@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sungem.h

@@ -953,6 +953,7 @@ enum link_state {
 
 struct gem {
 	spinlock_t lock;
+	spinlock_t tx_lock;
 	void __iomem *regs;
 	int rx_new, rx_old;
 	int tx_new, tx_old;