Crystal CS423x bugfixes

[win2kgamer]

Summary

Fixes some issues with the Crystal CS423x chips:

• Make reserved bit 4 of the I12 register read-only to resolve a CODEC detection issue in the OS/2 drivers
• I18/I19 now directly controls the FM volume on CS4236 non-B fixing the FM volume slider in the Windows drivers and DOS mixer
• Make reserved bits 7-1 of the I23 register read-only to resolve a detection issue with the CS4236 non-B in the Windows 2000 drivers
• Add the SBPro portion's missing music handler during init and always enable the OPL3 during context switching on CS4236 non-B since these chips lack the X registers (and IFM bit), both of these fix the OPL3 being silent in DOS

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/

References

Provide links to datasheets or other documentation that helped you implement this pull request.

• CS4232 datasheet (http://bitsavers.org/components/crystalSemiconductor/_dataSheets/CS4232.pdf)
• CS4236 datasheet (https://dosdays.co.uk/media/crystal/CS4236_datasheet.pdf)
• CS4236B datasheet (http://bitsavers.org/components/crystalSemiconductor/_dataSheets/CS4236B.pdf)
• CS4235 datasheet (http://bitsavers.org/components/crystalSemiconductor/_dataSheets/CS4235.pdf)