rtw88: follow the AP basic rates for tx mgmt frame

By default the driver uses the 1M and 6M rate for managemnt frames
in 2G and 5G bands respectively. But when the basic rates is
configured from the mac80211, we need to send the management frames
according to the basic rates.

This commit makes the driver use the lowest basic rates to send
the management frames.

Signed-off-by: Yu-Yen Ting <steventing@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/20211102022454.10944-1-pkshih@realtek.com