drivers/net/dnet.c: Fix compile warning

Fix this:
dnet.c: In function 'dnet_eth_initialize':
dnet.c:380:15: warning: assignment from incompatible pointer type

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>

drivers/net/dnet.c

@@ -130,8 +130,7 @@ static u16 dnet_mdio_read(struct dnet_device *dnet, u8 reg)
 	return value;
 }
 
-static int dnet_send(struct eth_device *netdev, volatile void *packet,
-		     int length)
+static int dnet_send(struct eth_device *netdev, void *packet, int length)
 {
 	struct dnet_device *dnet = to_dnet(netdev);
 	int i, wrsz;