[atl1e] Avoid unused variable warning in gcc 4.6

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/atl1e.c

 
 static void atl1e_init_ring_resources(struct atl1e_adapter *adapter)
 {
-	struct atl1e_tx_ring *tx_ring = NULL;
-	struct atl1e_rx_ring *rx_ring = NULL;
-
-	tx_ring = &adapter->tx_ring;
-	rx_ring = &adapter->rx_ring;
+	struct atl1e_rx_ring *rx_ring = &adapter->rx_ring;
 
 	rx_ring->real_page_size = adapter->rx_ring.page_size
 				 + MAX_FRAME_SIZE