netpoll: netpoll_poll_dev() should access dev->flags

commit 5a698af5 (bond: service netpoll arp queue on master device)
tested IFF_SLAVE flag against dev->priv_flags instead of dev->flags

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: WANG Cong <amwang@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>