drivers/net/rtl8169.c: Fix compile warning

Fix this:
rtl8169.c: In function 'rtl8169_initialize':
rtl8169.c:907:13: warning: assignment from incompatible pointer type
Signed-off-by: NJoe Hershberger <joe.hershberger@ni.com>

drivers/net/rtl8169.c

@@ -464,7 +464,7 @@ static int rtl_recv(struct eth_device *dev)
 /**************************************************************************
 SEND - Transmit a frame
 ***************************************************************************/
-static int rtl_send(struct eth_device *dev, volatile void *packet, int length)
+static int rtl_send(struct eth_device *dev, void *packet, int length)
 {
 	/* send the packet to destination */