tg3: Remove prev_vlan_tag from struct tx_ring_info

prev_vlan_tag field is not used.

Patch saves 512*8 bytes per tx queue ring on 64bit arches.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Acked-by: NMatthew Carlson <mcarlson@broadcom.com>

drivers/net/tg3.h

@@ -2412,7 +2412,6 @@ struct ring_info {
 
 struct tx_ring_info {
 	struct sk_buff			*skb;
-	u32				prev_vlan_tag;
 };
 
 struct tg3_config_info {