net/ncsi: Improve HNCDSC AEN handler

This improves AEN handler for Host Network Controller Driver Status
Change (HNCDSC):

   * The channel's lock should be hold when accessing its state.
   * Do failover when host driver isn't ready.
   * Configure channel when host driver becomes ready.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>