Fix a compiler warning in SIO code
Fix a compiler warning in SIO code

matthewbauer/mgba,sergiobenrocha2/mgba,fr500/mgba,fr500/mgba,fr500/mgba,Anty-Lemon/mgba,askotx/mgba,zerofalcon/mgba,Iniquitatis/mgba,nattthebear/mgba,libretro/mgba,AdmiralCurtiss/mgba,mgba-emu/mgba,libretro/mgba,zerofalcon/mgba,MerryMage/mgba,jeremyherbert/mgba,bentley/mgba,matthewbauer/mgba,jeremyherbert/mgba,Iniquitatis/mgba,Touched/mgba,MerryMage/mgba,cassos/mgba,askotx/mgba,iracigt/mgba,Anty-Lemon/mgba,nattthebear/mgba,askotx/mgba,sergiobenrocha2/mgba,Touched/mgba,iracigt/mgba,Iniquitatis/mgba,mgba-emu/mgba,zerofalcon/mgba,libretro/mgba,mgba-emu/mgba,jeremyherbert/mgba,mgba-emu/mgba,Touched/mgba,Anty-Lemon/mgba,Iniquitatis/mgba,fr500/mgba,cassos/mgba,sergiobenrocha2/mgba,cassos/mgba,jeremyherbert/mgba,sergiobenrocha2/mgba,iracigt/mgba,libretro/mgba,iracigt/mgba,MerryMage/mgba,AdmiralCurtiss/mgba,AdmiralCurtiss/mgba,askotx/mgba,sergiobenrocha2/mgba,libretro/mgba,bentley/mgba,Anty-Lemon/mgba