luci-app-ffwizard-falter: update to use option band instead of iwinfo

Instead of trying to figure out which freq a wireless device uses
based on it's reported freq list using iwinfo, simply use the "band"
option from the wireless config (since OpenWrt 21.02.2).  Also remove
deprecated hwmode option.

Signed-off-by: Perry Melange <isprotejesvalkata@gmail.com>

Author: pmelange

luci/luci-app-ffwizard-falter/luasrc/model/cbi/freifunk/assistent/wireless.lua

@@ -378,15 +378,14 @@ function calchwmode(device)
 end
 
 function getchannel(device)
-  local iwinfo = get_iwinfo(device)
-  local freqlist = iwinfo.freqlist(device)
+  local conf = uci:get("wireless", device, "band") or "X"
 
   local r_channel
-  if (freqlist[1].mhz > 2411 and freqlist[1].mhz < 2484) then
+  if (conf == "2g") then
     --this is 2.4 Ghz
     r_channel = tonumber(uci:get(community, "wifi_device", "channel")) or 13
   end
-  if (freqlist[1].mhz > 5179 and freqlist[1].mhz < 5701) then
+  if (conf == "5g") then
     --this is 5 Ghz
     r_channel = tonumber(uci:get(community, "wifi_device_5", "channel")) or 36
   end