WMF: fix initializing of media player's volume.

According to the docs, MESessionTopologyStatus with status == MF_TOPOSTATUS_READY should be the correct place for the GetService call.

Change-Id: I7fdbedbe43b2191b35b95c7fd9c86940f58daff7
Reviewed-by: Wouter Huysentruit <wouter_huysentruit@hotmail.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>