drivers/net/mpc512x_fec.c: Fix build warning

Fix:
mpc512x_fec.c: In function 'mpc512x_fec_initialize':
mpc512x_fec.c:634: warning: assignment from incompatible pointer type

Tested on mpc5121ads board using tftp and nfs commands.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Joe Hershberger <joe.hershberger@gmail.com>

drivers/net/mpc512x_fec.c

@@ -452,8 +452,8 @@ static void mpc512x_fec_halt (struct eth_device *dev)
 
 /********************************************************************/
 
-static int mpc512x_fec_send (struct eth_device *dev, volatile void *eth_data,
-		int data_length)
+static int mpc512x_fec_send(struct eth_device *dev, void *eth_data,
+			    int data_length)
 {
 	/*
 	 * This routine transmits one frame.  This routine only accepts