tg3: Avoid tx lockups on 5755+ devices

In certain edge conditions, internal tx resources can get corrupted.
This patch enables a bit that will fix the problem.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/tg3.h b/drivers/net/tg3.h
index 84ea0dca..c245e80 100644
--- a/drivers/net/tg3.h
+++ b/drivers/net/tg3.h
@@ -469,6 +469,7 @@
 #define  TX_MODE_FLOW_CTRL_ENABLE	 0x00000010
 #define  TX_MODE_BIG_BCKOFF_ENABLE	 0x00000020
 #define  TX_MODE_LONG_PAUSE_ENABLE	 0x00000040
+#define  TX_MODE_MBUF_LOCKUP_FIX	 0x00000100
 #define MAC_TX_STATUS			0x00000460
 #define  TX_STATUS_XOFFED		 0x00000001
 #define  TX_STATUS_SENT_XOFF		 0x00000002