RDMA/nes: Fix hangs on ifdown

When ib_unregister_device() is called from netdev stop during ifdown,
it sometimes hangs. Changes made to indicate port_err to ib_dispatch_event()
during netdev stop and port_active during netdev open. The
ib_unregister_device() is only called during remove of the module.

Signed-off-by: Faisal Latif <faisal.latif@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>