hostapd: add vendor_vht option

hostapd has vendor_vht option to enable VHT (256-QAM) on 2.4GHz
Add this option to hostapd.sh so users can enable it via uci

Signed-off-by: DENG Qingfang <dengqf6@mail2.sysu.edu.cn>

package/network/services/hostapd/files/hostapd.sh

@@ -116,6 +116,7 @@ hostapd_common_add_device_config() {
 	config_add_int rssi_reject_assoc_rssi
 	config_add_int rssi_ignore_probe_request
 	config_add_int maxassoc
+	config_add_boolean vendor_vht
 
 	config_add_string acs_chan_bias
 	config_add_array hostapd_options
@@ -133,7 +134,8 @@ hostapd_prepare_device_config() {
 
 	json_get_vars country country3 country_ie beacon_int:100 doth require_mode legacy_rates \
 		acs_chan_bias local_pwr_constraint spectrum_mgmt_required airtime_mode cell_density \
-		rts_threshold beacon_rate rssi_reject_assoc_rssi rssi_ignore_probe_request maxassoc
+		rts_threshold beacon_rate rssi_reject_assoc_rssi rssi_ignore_probe_request maxassoc \
+		vendor_vht
 
 	hostapd_set_log_options base_cfg
 
@@ -202,6 +204,7 @@ hostapd_prepare_device_config() {
 				set_default rate_list "24000 36000 48000 54000"
 				set_default basic_rate_list "24000"
 			fi
+			[ -n "$vendor_vht" ] && append base_cfg "vendor_vht=$vendor_vht" "$N"
 		;;
 		a)
 			if [ "$cell_density" -eq 1 ]; then