net: aquantia: oops when shutdown on already stopped device

In case netdev is closed at the moment of pci shutdown, aq_nic_stop
gets called second time. napi_disable in that case hangs indefinitely.
In other case, if device was never opened at all, we get oops because
of null pointer access.

We should invoke aq_nic_stop conditionally, only if device is running
at the moment of shutdown.

Reported-by: David Arcari <darcari@redhat.com>
Fixes: 90869ddf

 ("net: aquantia: Implement pci shutdown callback")
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>