nl80211: Try running without mgmt frame subscription (driver AP SME)

One of supported code paths already allows this scenario. It is used if
driver doesn't report NL80211_ATTR_DEVICE_AP_SME and doesn't support
monitor interface. In such situation:
1) We don't quit if subscribing for WLAN_FC_STYPE_PROBE_REQ fails
2) We don't try subscribing for WLAN_FC_STYPE_ACTION
3) We fallback to AP SME mode after failing to create monitor interface
4) We don't quit if subscribing for WLAN_FC_STYPE_PROBE_REQ fails
Above scenario is used, e.g., with brcmfmac. As you can see - thanks to
events provided by cfg80211 - it's not really required to receive Probe
Request or action frames.

However, the previous implementation did not allow using hostapd with
drivers that:
1) Report NL80211_ATTR_DEVICE_AP_SME
2) Don't support subscribing for PROBE_REQ and/or ACTION frames
In case of using such a driver hostapd will cancel setup after failing
to subscribe for WLAN_FC_STYPE_ACTION. I noticed it after setting flag
WIPHY_FLAG_HAVE_AP_SME in brcmfmac driver for my experiments.

This patch allows working with such drivers with just a small warning
printed as debug message.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

src/drivers/driver_nl80211.c

@@ -4302,7 +4302,8 @@ static int nl80211_setup_ap(struct i802_bss *bss)
 
 	if (drv->device_ap_sme && !drv->use_monitor)
 		if (nl80211_mgmt_subscribe_ap_dev_sme(bss))
-			return -1;
+			wpa_printf(MSG_DEBUG,
+				   "nl80211: Failed to subscribe for mgmt frames from SME driver - trying to run without it");
 
 	if (!drv->device_ap_sme && drv->use_monitor &&
 	    nl80211_create_monitor_interface(drv) &&