drivers/net/fec_mxc.c: Fix compile warning

Fix this:
fec_mxc.c: In function 'fec_probe':
fec_mxc.c:926:13: warning: assignment from incompatible pointer type

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

drivers/net/fec_mxc.c

@@ -687,7 +687,7 @@ static void fec_halt(struct eth_device *dev)
  * @param[in] length Data count in bytes
  * @return 0 on success
  */
-static int fec_send(struct eth_device *dev, volatile void *packet, int length)
+static int fec_send(struct eth_device *dev, void *packet, int length)
 {
 	unsigned int status;
 	uint32_t size;