resolve the test command for audiomixer service module

hi Edi

test the command "python3 -m pyagl.services.audiomixer 127.0.0.1 --setvolume 0.8", the result as below:

raspberrypi4-64:/usr/lib/python3.8/site-packages/pyagl/services# python3 -m pyagl.services.audiomixer 127.0.0.1 --setvolume 0.8
enter into audiomixer
Requesting list_controls with id 445798278
[RESPONSE][Status: success][445798278][Info: None][Data: [{'control': 'Master', 'volume': 0.6000000238418579, 'mute': 0}]]
Setting volume to 0.8 with id 26352007
[ERROR][Status: failed][26352007][Info: Could not find control][Data: None]

so, change the code， that‘s ok

python3 -m pyagl.services.audiomixer 127.0.0.1 --setvolume 0.6
enter into audiomixer
Requesting list_controls with id 4263204
[RESPONSE][Status: success][4263204][Info: None][Data: [{'control': 'Master', 'volume': 0.6000000238418579, 'mute': 0}]]
Setting volume to 0.6 with id 10098948
[RESPONSE][Status: success][10098948][Info: None][Data: {'volume': 0.6}]