wifi: rtl8xxxu: Add sta_add() and sta_remove() callbacks

In AP mode, sta_add() gets called when a new STA gets associated to
us. Call rtl8xxxu_refresh_rate_mask() to set a rate mask for the newly
connected STA (referenced by the macid) and then send a media connnect
report. Ignore the call to sta_add() in station mode.

Reserve one macid for broadcast/multicast packets in init.

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/20230428150833.218605-11-martin.kaistra@linutronix.de