wifi: rtl8xxxu: support multiple interfaces in set_aifs()

In concurrent mode supported by this driver, both interfaces will use
the same channel and same wireless mode.
It is therefore possible to get the wireless mode by checking the first
connected interface.

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/20231222101442.626837-11-martin.kaistra@linutronix.de