cnic: Defer iscsi connection cleanup

The bnx2x devices require a 2 second quiet time before sending the last
RAMROD command to destroy a connection.  This sleep wait adds up to a
long delay when iscsid is serially destroying maultiple connections.

Create a workqueue to perform the final connection cleanup in the
background to speed up the process.  This significantly speeds up the
process as the wait time can be done in parallel for multiple connections.

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