drivers/net/cs8900.c: Fix compile warning

Fix this:
cs8900.c: In function 'cs8900_initialize':
cs8900.c:326:12: warning: assignment from incompatible pointer type

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

drivers/net/cs8900.c

@@ -215,8 +215,7 @@ static int cs8900_recv(struct eth_device *dev)
 }
 
 /* Send a data block via Ethernet. */
-static int cs8900_send(struct eth_device *dev,
-			volatile void *packet, int length)
+static int cs8900_send(struct eth_device *dev, void *packet, int length)
 {
 	volatile u16 *addr;
 	int tmo;