cxgbe: Add a struct sge_ofld_txq type.
This type mirrors struct sge_ofld_rxq and holds state for TCP offload
transmit queues. Currently it only holds a work queue but will
include additional state in future changes.
Sponsored by: Chelsio Communications
(cherry picked from commit 077ba6a845fab8f1d3bd83e07f61730f202a46fc)