cnic: Send delete command when shutting down iSCSI ring.

This step is necessary on the bnx2x devices when restarting the iSCSI
ring.  Without it, the firmware can assert and cause bnx2x to report
errors.

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