wifi: rtl8xxxu: Remove usage of tx_info->control.rates[0].flags

As this driver uses HAS_RATE_CONTROL, rate_flags will not be provided by
mac80211.

Stop using tx_info->control.rates[0].flags and ieee80211_get_rts_cts_rate()
and use rts_threshold and bss_conf.use_cts_prot instead to determine
when to use RTS and CTS.

Send RTS with 24M rate like the vendor drivers. Also set this RTS rate
for ampdu_enable = true, because we also enable RTS for these frames.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230428150833.218605-17-martin.kaistra@linutronix.de