drivers/net/xilinx_axi_emac.c: Fix compile warning

Fix this:
xilinx_axi_emac.c: In function 'xilinx_axiemac_initialize':
xilinx_axi_emac.c:646: warning: assignment from incompatible pointer type

Signed-off-by: Stephan Linz <linz@li-pro.net>
diff --git a/drivers/net/xilinx_axi_emac.c b/drivers/net/xilinx_axi_emac.c
index 82e254b..7854a04 100644
--- a/drivers/net/xilinx_axi_emac.c
+++ b/drivers/net/xilinx_axi_emac.c
@@ -468,7 +468,7 @@
 	return 0;
 }
 
-static int axiemac_send(struct eth_device *dev, volatile void *ptr, int len)
+static int axiemac_send(struct eth_device *dev, void *ptr, int len)
 {
 	struct axidma_priv *priv = dev->priv;
 	u32 timeout;