FW has some usecases when it needs to enqueue frames into hardware. In
BE WLAN chips, some hardware TX desc fields are moved into Tx
banks, hence a TX bank is also needed to queue to HW.
Dedicate a bank for FW usage.
Change-Id: I1f19f0ef85aff4c7592b0f07de4631259da743a6
CRs-Fixed: 3016828