Not able to adjust Audio input UI volume after plugging headset when current page is Sound->input tab

In some situations which only has one input item in the treeview, audio input volume can't be adjusted(gray out).

Prerequisite:

1. If there is only one input device with only one input port, and this input port is active_port, e.g. there is no internal mic, just a mic jack on the machine, this issue will happen.
2. In this machine won't show "selection icon" when plugging input audio device (some machines will show this selection icon, but some doesn't)

Steps to reproduce:

1. Open GNOME Settings
2. Change tab to Sound-> Input
3. Plug headset to headset jack (meant you can't plug your headset before toggle to Sound/Sound->Input tab).
4. Device shows up as below picture, but can't adjust it's volume, in the meantime syslog shows:

'''
gnome-control-c[4446]: No devices in the tree, so cannot set the active output
Unable to get default source
'''

Some Other Info:

1. For this machine, if users plug mic in the jack, the function active_input_update() is called ahead of the function add_input_ui_entry(), then we will see "No devices in the tree, so cannot set the active output" from syslog, this issue will be reproduced.

2. On other machines, the add_input_ui_entry() is called ahead of active_input_update(), then there is no problem.