wifi: wfx: repair open network AP mode

RSN IE missing in beacon is normal in open networks.
Avoid returning -EINVAL in this case.

Steps to reproduce:

$ cat /etc/wpa_supplicant.conf
network={
	ssid="testNet"
	mode=2
	key_mgmt=NONE
}

$ wpa_supplicant -iwlan0 -c /etc/wpa_supplicant.conf
nl80211: Beacon set failed: -22 (Invalid argument)
Failed to set beacon parameters
Interface initialization failed
wlan0: interface state UNINITIALIZED->DISABLED
wlan0: AP-DISABLED
wlan0: Unable to setup interface.
Failed to initialize AP interface

After the change:

$ wpa_supplicant -iwlan0 -c /etc/wpa_supplicant.conf
Successfully initialized wpa_supplicant
wlan0: interface state UNINITIALIZED->ENABLED
wlan0: AP-ENABLED

Cc: stable@vger.kernel.org
Fixes: fe0a777 ("wifi: wfx: fix possible NULL pointer dereference in wfx_set_mfp_ap()")
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20240823131521.3309073-1-alexander.sverdlin@siemens.com

drivers/net/wireless/silabs/wfx/sta.c

@@ -352,8 +352,11 @@ static int wfx_set_mfp_ap(struct wfx_vif *wvif)
 
 	ptr = (u16 *)cfg80211_find_ie(WLAN_EID_RSN, skb->data + ieoffset,
 				      skb->len - ieoffset);
-	if (unlikely(!ptr))
+	if (!ptr) {
+		/* No RSN IE is fine in open networks */
+		ret = 0;
 		goto free_skb;
+	}
 
 	ptr += pairwise_cipher_suite_count_offset;
 	if (WARN_ON(ptr > (u16 *)skb_tail_pointer(skb)))