rtw88: add debugfs to force lowest basic rate

The management frame with high rate e.g. 24M may not be transmitted
smoothly in long range environment.
Add a debugfs to force to use the lowest basic rate
in order to debug the reachability of transmitting management frame.

obtain current setting
cat /sys/kernel/debug/ieee80211/phyX/rtw88/force_lowest_basic_rate

force lowest rate:
echo 1 > /sys/kernel/debug/ieee80211/phyX/rtw88/force_lowest_basic_rate

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/20211102022454.10944-2-pkshih@realtek.com