fix volume_status for default source with pamixer

[oceyral]

Hi,

#2228 and #2230 introduced a regression in volume_status when using it with the default source.

Example configuration chunk :

order += "volume_status mic"

volume_status "mic" {
      is_input = True
      cache_timeout = 1
}

With this configuration, instead of selecting the default source, volume_status selects the default sink by calling pamixer --allow-boost --get-mute --get-volume

This PR adds --default-source to the command in case no device is specified and the option is_input is True.

[syyyr]

Sorry for the regression. I have tested this patch, and my issue (the one I addressed in the PRs) hasn't reappeared.

[ultrabug]

Thanks!