hostapd: For VHT 20/40, allow center segment 0 to be zero

The 802.11ac amendment specifies that that the center segment 0 field
is reserved, so it should be zero. Hostapd previously required it to
be set, which is likely a good idea for interoperability, but allow it
to be unset. However, don't allow it to be set to a random value, only
allow zero and the correct channel.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

src/ap/ap_drv_ops.c

@@ -503,7 +503,8 @@ int hostapd_set_freq_params(struct hostapd_freq_params *data, int mode,
 	case VHT_CHANWIDTH_USE_HT:
 		if (center_segment1)
 			return -1;
-		if (5000 + center_segment0 * 5 != data->center_freq1 &&
+		if (center_segment0 != 0 &&
+		    5000 + center_segment0 * 5 != data->center_freq1 &&
 		    2407 + center_segment0 * 5 != data->center_freq1)
 			return -1;
 		break;