wifi: rtl8xxxu: Remove usage of ieee80211_get_tx_rate()

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

For some frames c->control.rates[0].idx is negative, which means
ieee80211_get_tx_rate() will print a warning and return NULL.

Only management frames have USE_DRIVER_RATE set, so for all others the
rate info of txdesc is ignored anyway.

Remove call to ieee80211_get_tx_rate() and send management frames with
1M (rate info = 0).

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/20230428150833.218605-16-martin.kaistra@linutronix.de