gluon-luci-wifi-config: don't use luci.sys.wifi.getiwinfo()

Directly using libiwinfo is more reliable.

package/gluon-luci-wifi-config/files/usr/lib/lua/luci/model/cbi/admin/wifi-config.lua

 local uci = luci.model.uci.cursor()
 local fs = require 'nixio.fs'
+local iwinfo = require 'iwinfo'
 
 
 local function find_phy_by_path(path)
@@ -17,9 +18,9 @@ local function find_phy_by_macaddr(macaddr)
   end
 end
 
-local function txpower_list(iw)
-  local list = iw.txpwrlist or { }
-  local off  = tonumber(iw.txpower_offset) or 0
+local function txpower_list(phy)
+  local list = iwinfo.nl80211.txpwrlist(phy) or { }
+  local off  = tonumber(iwinfo.nl80211.txpower_offset(phy)) or 0
   local new  = { }
   local prev = -1
   local _, val
@@ -93,9 +94,7 @@ for _, radio in ipairs(radios) do
     end
 
     if phy then
-      local iw = luci.sys.wifi.getiwinfo(phy)
-      if iw then
-        local txpowers = txpower_list(iw)
+      local txpowers = txpower_list(phy)
 
       if #txpowers > 1 then
         local tp = p:option(ListValue, radio .. '_txpower', translate("Transmission power"))
@@ -112,7 +111,6 @@ for _, radio in ipairs(radios) do
       end
     end
   end
-  end
 
 end