alsa-gobject: normalize function signatures

[GitHub pull_request - opened]

alsa-project/alsa-gobject pull request #11 was opened from takaswie:

The alsa-gobject project has a loose convention to have functions returning nothing. This pull
request refine according to it as well as fixes for some bugs.

Request URL   : https://github.com/alsa-project/alsa-gobject/pull/11
Patch URL     : https://github.com/alsa-project/alsa-gobject/pull/11.patch
Repository URL: https://github.com/alsa-project/alsa-gobject