wifi: rtw89: refine 6 GHz scanning dwell time

Reduce dwell time to improve scan duration in 6 GHz. This is required
for scan requests that does not include RNR parsing and does full
channel scan.

Signed-off-by: Po-Hao Huang <phhuang@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/20221214091952.42792-1-pkshih@realtek.com