cxgbe/t4_tom: Support for round-robin selection of offload queues.

A COP (Connection Offload Policy) rule can now specify that the tx
and/or rx queue for a new tid should be selected in a round-robin
manner. There is no change in default behavior.

Reviewed by: jhb@
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34921