[NETPOLL]: wrong return for null netpoll_poll_lock()

When netpoll is not being used, the macro that
defines the removed routing netpoll_poll_lock
defines the return as zero, but the real
routine returns a `void *`

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netpoll.h

@@ -86,7 +86,7 @@ static inline void netpoll_poll_unlock(void *have)
 
 #else
 #define netpoll_rx(a) 0
-#define netpoll_poll_lock(a) 0
+#define netpoll_poll_lock(a) NULL
 #define netpoll_poll_unlock(a)
 #endif