AD1816 fixes

[win2kgamer]

Summary

Make the following changes to the AD1816:

• Don't reset playback_pos (byte counter) when sample count drops below 0, fixes some playback glitches on the Windows 3.1 drivers
• Right shift the wave and master volume values by 1 to make wave output audible on Windows 3.1 with the default mixer volume values. Also makes the mid-point wave volume setting usable on other drivers.

Checklist

• Closes Analog devices AD1816 no sound effects under Windows 3.1 #6626
• 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.