Changes bandwidth of 2.4GHz channels from 40MHz to 20MHz (HT20).

Since a "dual channel" 40MHz bandwidth is barely usable with 2.4GHz
channels in urban environments due to a huge amounts of interfering
other wireless networks, we have to restrict our bandwidth usage to
20MHz to prevent too many collisions with other networks. As collisions
result in retransmissions, this makes the WiFi-links less attractive for
B.A.T.M.A.N., resulting in either the "loss" of certain WiFi-links or
the preference of the VPN-link. Neither one is really desired, hence
taking the bitter pill of loosing about half the bandwidth of the links
(optimally 300MBit/s on 40MHz, 144.4MBit/s on 20MHz) should result in a
better mesh-network performance.

Since the 5GHz-band is still rather uncrowed today and much broader
anyway, the bandwith for those channels remain at 40MHz.

site.conf

@@ -32,7 +32,7 @@
 	wifi24 = {
 		ssid = 'paderborn.freifunk.net',
 		channel = 1,
-		htmode = 'HT40+',
+		htmode = 'HT20',
 		mesh_ssid = '02:d1:11:13:87:ad',
 		mesh_bssid = '02:d1:11:13:87:ad',
 		mesh_mcast_rate = 12000,