wifi: wfx: repair open network AP mode

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: fe0a7776d4d1 ("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