netvsc: use typed pointer for internal state

The element netvsc_device:extension is always a pointer to RNDIS
information.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/hyperv_net.h

@@ -763,8 +763,7 @@ struct netvsc_device {
 
 	refcount_t sc_offered;
 
-	/* Holds rndis device info */
-	void *extension;
+	struct rndis_device *extension;
 
 	int ring_size;