rtw88: not to enter or leave PS under IRQ

Remove PS related *_irqsafe functions to avoid entering/leaving PS
under interrupt context. Instead, make PS decision in watch_dog.
This could simplify the logic and make the code look clean.

But it could have a little side-effect that if the driver is having
heavy traffic before the every-2-second watch_dog detect the traffic
and decide to leave PS, the thoughput will be lower. Once traffic is
detected by watch_dog and left PS state, the throughput will resume
to the peak the hardware ought to have again.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>