wifi: rtl8xxxu: fix mixed declarations in rtl8xxxu_set_aifs()

Moving struct ieee80211_sta *sta variable definition to the front
of the code to fix the ISO C90 forbids mixed declarations and code
warning.

Fixes: 43532c05 ("wifi: rtl8xxxu: support multiple interfaces in set_aifs()")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/TYAP286MB03157A408E0D69F2F6FBD88ABC552@TYAP286MB0315.JPNP286.PROD.OUTLOOK.COM

drivers/net/wireless/realtek/rtl8xxxu/rtl8xxxu_core.c

@@ -4919,11 +4919,11 @@ static void rtl8xxxu_set_aifs(struct rtl8xxxu_priv *priv, u8 slot_time)
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(priv->vifs); i++) {
+		struct ieee80211_sta *sta;
+
 		if (!priv->vifs[i])
 			continue;
 
-		struct ieee80211_sta *sta;
-
 		rcu_read_lock();
 		sta = ieee80211_find_sta(priv->vifs[i], priv->vifs[i]->bss_conf.bssid);
 		if (sta)