tg3: Add prodring parameter to tg3_alloc_rx_skb()

This patch changes the tg3_alloc_rx_skb() implementation to accept the
destination producer ring set pointer as a parameter rather than
assuming the source and destination producer rings are the same.

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