In the case of WCN6450, WBM HW block is removed in the UMAC.
TX completions come via HTT messages. Add logic to handle
HTT TX completion messages from the firmware.
Changes are specific to WCN6450 and hence implement the logic
in the arch specific code.
Change-Id: I447020354ce26e8948e4b49648c434fb2ed302cd
CRs-Fixed: 3381814