wifi: rtw88: add flushing queue before HW scan

We need to flush queue before HW scan to avoid packets dropped by hardware.

Signed-off-by: Chih-Kang Chang <gary.chang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220809084107.38137-6-pkshih@realtek.com