net: Use local variable devices instead of net.devices

[greenfork]

Using widget:set_markup in the setting function triggers the following error:

/usr/share/awesome/lib/gears/object.lua:189: Cannot set 'devices' on
wibox.widget.textbox (table: 0xXXXXX) because it is read-only.

This happens because of the definition of net:

local net = { widget = args.widget or wibox.widget.textbox(), devices: {} }

The error complains about the devices field.

Everything works with widget:set_text, I suppose set_markup needs all the fields in the widget.

I have moved the table field to a local variable to allow using it with set_markup.