[netdrvr] mv643xx_eth: fix obvious typo, which caused build breakage

The last minute fix submitted by the author fixed a bug, but
broke the driver build.

Noticed by Al Viro, since I can't build on said platform.
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/mv643xx_eth.c

@@ -385,7 +385,7 @@ static int mv643xx_eth_receive_queue(struct net_device *dev, int budget)
 	struct pkt_info pkt_info;
 
 	while (budget-- > 0 && eth_port_receive(mp, &pkt_info) == ETH_OK) {
-		dma_unmap_single(NULL, pkt_info.buf_ptr, RX_SKB_SIZE,
+		dma_unmap_single(NULL, pkt_info.buf_ptr, ETH_RX_SKB_SIZE,
 							DMA_FROM_DEVICE);
 		mp->rx_desc_count--;
 		received_packets++;