Summary

Make the following changes to the CS423x:

• Make the lower 3 bits of CS423x control register C0 (WSS Master Control) writable on CS4236B and later, resolves the "invalid codec" error in Win3.1 v2.x drivers, the control register code 10 on the Win9x v2.86 drivers and corrects some mixer behavior (Win9x drivers show more volume sliders and now use the X6/X7 registers for synth volume when FM remap is disabled.)
• Remove the old CS4236B+ I18/I19 mixer hack as it is no longer needed (and causes issues with the Win3.1 v1.82 driver after the change above.)

Checklist

• Closes Crystal CS4235 does not work with Windows 3.1 Crystal driver version 2.00 #6771
• 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.