GB Audio: Fix channel 1 shadow register
GB Audio: Fix channel 1 shadow register

Iniquitatis/mgba,sergiobenrocha2/mgba,libretro/mgba,jeremyherbert/mgba,Anty-Lemon/mgba,fr500/mgba,fr500/mgba,jeremyherbert/mgba,fr500/mgba,iracigt/mgba,sergiobenrocha2/mgba,Touched/mgba,mgba-emu/mgba,jeremyherbert/mgba,Iniquitatis/mgba,Anty-Lemon/mgba,Anty-Lemon/mgba,sergiobenrocha2/mgba,sergiobenrocha2/mgba,Touched/mgba,mgba-emu/mgba,Iniquitatis/mgba,Iniquitatis/mgba,libretro/mgba,libretro/mgba,jeremyherbert/mgba,libretro/mgba,mgba-emu/mgba,MerryMage/mgba,iracigt/mgba,MerryMage/mgba,sergiobenrocha2/mgba,iracigt/mgba,mgba-emu/mgba,iracigt/mgba,MerryMage/mgba,libretro/mgba,Touched/mgba,Anty-Lemon/mgba,fr500/mgba