rtw88: more descriptions about LPS

The LPS represents Leisure Power Save. When enabled, firmware will be in
charge of turning radio off between beacons. Also firmware should turn
on the radio when beacon is coming, and the data queued should be
transmitted in TBTT period.

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