davinci: Fix Ethernet driver interface warning

Fixes:
davinci_emac.c: In function 'davinci_emac_initialize':
davinci_emac.c:796:12: warning: assignment from incompatible pointer type [enabled by default]

Tested on da850_am18xxevm

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

drivers/net/davinci_emac.c

@@ -637,7 +637,7 @@ static int tx_send_loop = 0;
  * positive number (number of bytes transmitted) or negative for error
  */
 static int davinci_eth_send_packet (struct eth_device *dev,
-					volatile void *packet, int length)
+					void *packet, int length)
 {
 	int ret_status = -1;
 	int index;