dpif-netdev: Fail port addition if reconfiguration failed.

dpif-netdev: Fail port addition if reconfiguration failed.

If the port was destroyed during the initial reconfiguration, we should
report an error to the upper layers. Otherwise, successful addition of
the port will be logged and upper layers will continue to configure
this port. For example, the 'dpif' layer will try to initilaize flow
API for this device.

Fix that by checking for port existence after reconfiguration. We can't
get the real error code here, so let's assume EINVAL. 'ovs-vsctl' will
tell the user to check the logs for a real reason anyway.

Fixes: e32971b8ddb4 ("dpif-netdev: Centralized threads and queues handling code.")
Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Acked-by: Ian Stokes <ian.stokes@intel.com>