ath10k: handle mgmt tx completion event

WCN3990 transmits management frames via WMI
with reference. Currently, with the management
tx completion not being handled, these frames are
not getting freed even after the transmission status
is returned by the firmware.

The transmitted management frames should be freed
when the firmware sends the over-the-air tx status of
the corresponding management frames.

Handle the wmi mgmt tx completion event and free
the corresponding management frame.

Tested HW: WCN3990
Tested FW: WLAN.HL.2.0-01188-QCAHLSWMTPLZ-1

Signed-off-by: Rakesh Pillai <pillair@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>