cnic: Refine registration with bnx2.

Register and unregister with bnx2 during NETDEV_UP and NETDEV_DOWN
events.  This simplifies the sequence of events and allows locking
fixes in the next patch.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>