rtw88: add flush hci support

Though mac queue flushing has been supported, sometimes data may be waiting
on interface from host to chip. If it occurs, there may still be data that
flows into mac just after we do flush. To avoid that, we add the hci part
of flushing.

Signed-off-by: Zong-Zhe Yang <kevin_yang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210319054218.3319-2-pkshih@realtek.com