win-dshow: Set audio only active when there is audio

[exeldro]

Description

Set audio on dshow_input only active when there is audio

Motivation and Context

I don't want devices in the audio mixer that don't have audio. My C910 webcam by default has no audio when added in OBS, but it shows in the audio mixer.

How Has This Been Tested?

On windows 10 64bit with my C910

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.