https://www.brunsware.de/blog/hostapd-40mhz-disable-neighbor-check

--- hostapd-2.9/src/ap/hw_features.c
+++ hostapd-2.9/src/ap/hw_features.c
@@ -310,7 +310,7 @@
 	wpa_scan_results_free(scan_res);
 
 	iface->secondary_ch = iface->conf->secondary_channel;
-	if (!oper40) {
+/*	if (!oper40) {
 		wpa_printf(MSG_INFO, "20/40 MHz operation not permitted on "
 			   "channel pri=%d sec=%d based on overlapping BSSes",
 			   iface->conf->channel,
@@ -318,14 +318,14 @@
 			   iface->conf->secondary_channel * 4);
 		iface->conf->secondary_channel = 0;
 		if (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX) {
-			/*
+*/			/*
 			 * TODO: Could consider scheduling another scan to check
 			 * if channel width can be changed if no coex reports
 			 * are received from associating stations.
 			 */
-		}
+/*		}
 	}
-
+*/
 	res = ieee80211n_allowed_ht40_channel_pair(iface);
 	if (!res) {
 		iface->conf->secondary_channel = 0;
--- hostapd-2.9/src/ap/ieee802_11_ht.c
+++ hostapd-2.9/src/ap/ieee802_11_ht.c
@@ -349,7 +349,7 @@
 	wpa_printf(MSG_DEBUG, "is_ht40_allowed=%d num_sta_ht40_intolerant=%d",
 		   is_ht40_allowed, iface->num_sta_ht40_intolerant);
 
-	if (!is_ht40_allowed &&
+/*	if (!is_ht40_allowed &&
 	    (iface->drv_flags & WPA_DRIVER_FLAGS_HT_2040_COEX)) {
 		if (iface->conf->secondary_channel) {
 			hostapd_logger(hapd, mgmt->sa,
@@ -372,7 +372,7 @@
 				   "Reschedule HT 20/40 timeout to occur in %u seconds",
 				   delay_time);
 		}
-	}
+	}*/
 }