[16-bit Multiplayer]: Transfers contain repeated slave data

[afska]

LinkRawCable_demo-v7.zip

When using this ROM, slave data is never repeated on hardware (REG_SIOMULTI1 returns 0xFFFF)

hardware.mp4

On mGBA's development version (0.11-8709-f108dd801) and old stable versions too, player 0 gets the latest value sent by player 1 every time it sends.

mgbadev.mp4

I'm not sure if this is a limitation of the current system but since I faced some bugs related to this, I thought it'd be a good thing to report it!