[Bug] Fix Valgrind reports memory corruption when Setting the Mute property

- Valgrind reports memory corruption when Setting the Mute property.
- If you set the Mute value, the received last change event only contains
a value for mute. It does not contain a value for Volume, and so
device_volume was not initialized.
- The fix consists in initializing device_volume with G_MAXUINT, so we can
check if the volume was part of the received last change event.
- Fix issue: https://github.com/01org/dleyna-renderer/issues/37

Signed-off-by: Christophe Guiraud <christophe.guiraud@intel.com>