wifi: rtl8xxxu: update rate mask per sta

Until now, rtl8xxxu_watchdog_callback() only fetches RSSI and updates
the rate mask in station mode. This means, in AP mode only the default
rate mask is used.

In order to have the rate mask reflect the actual connection quality,
extend rtl8xxxu_watchdog_callback() to iterate over every sta. Like in
the rtw88 driver, add a function to collect all currently present stas
and then iterate over a list of copies to ensure no RCU lock problems
for register access via USB. Remove the existing RCU lock in
rtl8xxxu_refresh_rate_mask().

Since the currently used ieee80211_ave_rssi() is only for 'vif', add
driver-level tracking of RSSI per sta.

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://msgid.link/20240205093040.1941140-1-martin.kaistra@linutronix.de