rtw88: 8723d: implement flush queue

Flush queue is used to check if queue is empty, before doing something
else. Since 8723D uses different registers and page number of
availabl/reserved occupy 8 bits instead of 16 bits, so use a 'wsize' field
to discriminate which rtw_read{8,16} is adopted.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200512102621.5148-6-yhchuang@realtek.com