LE Audio: Replace invalid VolumeControl::Get use by IsVolumeControlRunning

VolumeControl::Get asserts if the instance is null,
comparing it to nullptr is invalid as this case is imposible.
Use VolumeControl::IsVolumeControlRunning that do the proper check instead.

Change-Id: Ife3edd2545c0740fe948f2ea851ee4f1ed1b2177
Test: TreeHugger