bnx2x, cnic: Disable iSCSI if DCBX negotiation is successful

With current bnx2x firmware 6.2.9, iSCSI is not supported in DCB
network, so we need to disable it.  Add cnic command to disconnect
iSCSI connections and prevent future connections when DCBX negotiation
succeeds.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>