wifi: rtl8xxxu: prepare supporting two virtual interfaces

To prepare for concurrent mode, add an array ("vifs") to rtl8xxxu_priv
to keep track of both interfaces.

Keep the old priv->vif as long there are still users of it and let
priv->vifs[0] point to the same location.

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/20231222101442.626837-3-martin.kaistra@linutronix.de