tg3: Cleanup tg3_alloc_rx_skb()

src_map is no longer used in tg3_alloc_rx_skb().  Remove it.

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