wifi: rtl8xxxu: Set maximum number of supported stations

Set maximum number of associated stations supported in AP mode. For
8188f, the maximum number of supported macids is 16, reserve one for
broadcast/multicast 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/20230428150833.218605-19-martin.kaistra@linutronix.de