AESinkPULSE: Revisit volume

As xbmc internall uses Scaled Gain, DB and a lot of other stuff, changing volume was quite fine, but not setting the volume to a definite value, which was needed from the outside. 

The volume bar will not move really linear anymore, but as the volume is changed with db values - that's quite clear. I am doing the same workaround for volume as on IOS

Was tested by @wsnipex