ravb: fix DMA channel misreporting

Currently 'ifconfig' for the Ethernet devices handled by this driver  shows
"DMA chan: ff" while the driver doesn't use any DMA channels. Not assigning
a value to 'net_device::dma' causes 'ifconfig'  to  correctly not report  a
DMA channel.
Signed-off-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/renesas/ravb_main.c

@@ -1908,7 +1908,6 @@ static int ravb_probe(struct platform_device *pdev)
 
 	/* The Ether-specific entries in the device structure. */
 	ndev->base_addr = res->start;
-	ndev->dma = -1;
 
 	chip_id = (enum ravb_chip_id)of_device_get_match_data(&pdev->dev);