drivers/net/natsemi.c: fix compile warning

Fix warning: natsemi.c:757: warning: dereferencing type-punned pointer
will break strict-aliasing rules

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Ben Warren <biggerbadderben@gmail.com>

drivers/net/natsemi.c

@@ -754,7 +754,8 @@ natsemi_send(struct eth_device *dev, volatile void *packet, int length)
 {
 	u32 i, status = 0;
 	u32 tx_status = 0;
-	vu_long *res = (vu_long *)&tx_status;
+	u32 *tx_ptr = &tx_status;
+	vu_long *res = (vu_long *)tx_ptr;
 
 	/* Stop the transmitter */
 	OUTL(dev, TxOff, ChipCmd);