Psg: reenable and fix "fixed volume"
Psg: reenable and fix "fixed volume"

Problem was simply that I was reading the wrong aplitude control register indices because they're listed in octal in the data sheet, so R10/11/12 are actually R8/9/10 in decimal.

amaiorano/vectrexy,amaiorano/vectrexy,amaiorano/vectrexy