Summary

Fix the following on the original Pro Audio Spectrum:

• YM3802 Interrupt Vector register should have bit 4 set when no interrupts are active, fixes playback in the PAS demo and Voyetra sequencer
• Reset the YM3802 registers when bit 7 is set then unset in the System Control register
• Further corrections to YM3802 IVR and ISR register handling, fixes unreliable MIDI input in the Voyetra sequencer and Windows 3.1

Checklist

• Closes #xxx
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/
• This pull request requires changes to the asset set
  • I have opened an assets pull request - https://github.com/86Box/assets/pull/changeme/

References

Provide links to datasheets or other documentation that helped you implement this pull request.
Yamaha YM3802 MCS Application Manual: https://www.msxarchive.nl/pub/msx/docs/datasheets/yamahaym3802am.pdf