AESinkPULSE: Fix linear volume to do the same as other PA apps

It's not quite easy to get the pavolume scaling correctly. After some forum reports I decided to make that more usable from a user point of view and make it the same as pavucontrol does. Now 50% in xbmc is equivalent to 50% in pavucontrol.

In my older code I used to make 50% == half the volume, but users did not like that approach :-)