v4l2: support radio devices in device monitor

Submitted by bud..@..il.com

Link to original bug (#743108)

Description

I want to create a filter to monitored v4l2 radio device in my system using GstDeviceMonitor.
With actual API, can monitor a v4l2 device with tv tuner but do not know if it is possible to create a filter for radio tuner.

Version: 1.4.5