rtw88: sync the power state between driver and firmware

In current flow, driver issues power requests to firmware for entering or
leaving deep power mode. But driver does not wait for an ack from firmware
via reading CPWM register when driver requests to enter deep power mode.
The behavior will lead to unsynchronized state between firmware and driver.
Furthermore, consecutive same power requests may confuse firmware and leads
to unexpected value of CPWM register. This patch ensures that the power
request will wait for an ack from firmware and only send one power request
each time.

Signed-off-by: Chin-Yen Lee <timlee@realtek.com>
Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201030084826.9034-2-tehuang@realtek.com