Virtual Boy: fix modulation bug & limit sample rate

[skyfloogle]

This pull request includes 2 changes:

• Fixes a bug I introduced in Improve Virtual Boy emulator #2446. I implemented a hardware bug where writing to S5FQL or S5FQH locks the corresponding byte of the frequency value. In my initial implementation, however, this triggered when writing the FQL or FQH registers of any channel, not just channel 5.
• Emits 41666.666 samples per second. This corresponds to behaviour identified by yupferris here, which I have independently verified. As a result, sounds with a higher pitched will be aliased, as they would be on hardware.

Reproducing samples with before/after/hardware for each (may not be pleasant to listen to):
repros.zip