tg3: sparse cleanup

Fix the following sparse warning :

drivers/net/tg3.c:4025:3: warning: context imbalance in 'tg3_restart_hw' 
- unexpected unlock
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tg3.c

@@ -4017,6 +4017,8 @@ static int tg3_halt(struct tg3 *, int, int);
  * Invoked with tp->lock held.
  */
 static int tg3_restart_hw(struct tg3 *tp, int reset_phy)
+	__releases(tp->lock)
+	__acquires(tp->lock)
 {
 	int err;