drivers/net/lan91c96.c: Fix compile warning

Fix this:
lan91c96.c: In function 'lan91c96_initialize':
lan91c96.c:811:12: warning: assignment from incompatible pointer type

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

drivers/net/lan91c96.c

@@ -314,7 +314,7 @@ static void smc_shutdown(struct eth_device *dev)
  *	Enable the transmit interrupt, so I know if it failed
  *	Free the kernel data if I actually sent it.
  */
-static int smc_send_packet(struct eth_device *dev, volatile void *packet,
+static int smc_send_packet(struct eth_device *dev, void *packet,
 		int packet_length)
 {
 	byte packet_no;
@@ -700,7 +700,7 @@ static int lan91c96_recv(struct eth_device *dev)
 	return smc_rcv(dev);
 }
 
-static int lan91c96_send(struct eth_device *dev, volatile void *packet,
+static int lan91c96_send(struct eth_device *dev, void *packet,
 		int length)
 {
 	return smc_send_packet(dev, packet, length);