tg3: Make tg3_alloc_rx_skb() a dst-only operation

This patch removes the source index parameter of tg3_alloc_rx_skb().  A
later patch will make it possible for the source and destination
producer rings to be different.  This patch opts to make
tg3_alloc_rx_skb() a destination-only implementation and move the code
sensitive to the difference elsewhere.

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