wifi: rtl8xxxu: 8188e: convert usage of priv->vif to priv->vifs[0]

The driver currently does not support AP or concurrent mode for 8188e,
so just use priv->vifs[0] instead of priv->vif for now.

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/20231222101442.626837-5-martin.kaistra@linutronix.de