tg3: Make the TX BD DMA limit configurable

The 57766 ASIC rev will impose a new TX BD DMA limit on the driver.
This patch prepares for 57766 support by making the tx BD DMA limit
tunable.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/tg3.h

@@ -2994,6 +2994,7 @@ struct tg3 {
 	/* begin "tx thread" cacheline section */
 	void				(*write32_tx_mbox) (struct tg3 *, u32,
 							    u32);
+	u32				dma_limit;
 
 	/* begin "rx thread" cacheline section */
 	struct tg3_napi			napi[TG3_IRQ_MAX_VECS];