[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>