drivers/net/enc28j60.c: Fix compile warning

Fix this:
enc28j60.c: In function 'enc28j60_initialize':
enc28j60.c:967:12: warning: assignment from incompatible pointer type

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

drivers/net/enc28j60.c

@@ -875,7 +875,7 @@ static int enc_recv(struct eth_device *dev)
  */
 static int enc_send(
 	struct eth_device *dev,
-	volatile void *packet,
+	void *packet,
 	int length)
 {
 	enc_dev_t *enc = dev->priv;