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>
diff --git a/drivers/net/dnet.c b/drivers/net/dnet.c
index 15d0a6e..944a0c0 100644
--- a/drivers/net/dnet.c
+++ b/drivers/net/dnet.c
@@ -130,8 +130,7 @@
 	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;