cxgb4: Add routines to create and remove listening IPv6 servers

Add cxgb4_create_server6 and cxgb4_remove_server routines to create
and remove listening IPv6 servers.

Return success (0) from cxgb4_create_server in case of ctrl queue
congestion since in case of congestion, passive open request gets
queued and gets processed later.  If a non-zero value were returned it
would be treated as an error and the ULD would free STID, which can
result in an error in passive open reply.

Add cpl structure for active open request with IPv6 address for T5.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>