wcn36xx: Send NULL data packet when exiting BMPS

This commit updates the BMPS exit path to be consistent with downstream in
terms of exiting BMPS mode. Downstream sets the flag to send a NULL data
frame to the host on exiting BMPS.

This will tell the AP to send any queued frames to the STA immediately.
Verified the relevant bit toggle in wireshark.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201120021403.2646574-2-bryan.odonoghue@linaro.org