ath10k: add waiting htt tx complete before wow enable

If there are some tx packets pending in firmware, and then system
enters suspend, firmware will fail for wow enable. This will trigger
mac80211 to stop ath10k and download firmware again, then it is
non-wow suspend.

After add the waiting htt tx complete, then firmware will have some
time window to send or flush the pending tx packets.

Tested with QCA6174 PCI with firmware
WLAN.RM.4.4.1-00109-QCARMSWPZ-1, but this will also affect QCA9377 PCI.
It's not a regression with new firmware releases.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>