QUPowerDeviceInterface: Fix a memory leak.

Change-Id: I5e671429dcfd2968ccf5153601b0e8855cf0482b
Done-by: Mohammed Sameer <mohammed.hassan@jolla.com>
Reviewed-by: Mohammed Sameer <msameer@foolab.org>
Reviewed-by: Lorn Potter <lorn.potter@jollamobile.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>