NET: netpoll, fix potential NULL ptr dereference

Stanse found that one error path in netpoll_setup dereferences npinfo
even though it is NULL. Avoid that by adding new label and go to that
instead.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Daniel Borkmann <danborkmann@googlemail.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by:  <chavey@google.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>