netpoll: call dev_put() on error in netpoll_setup()

There is a dev_put(ndev) missing on an error path.  This was
introduced in 0c1ad04a

 "netpoll: prevent netpoll setup on slave
devices".

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>