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.

Doing so will immediately indicate to the host that the device is alive.
This is the right thing to do when doing firmware offloaded keepalive since
we may have been off channel when the firmware's timer to send the NULL
data-frame expired.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>