ChibiOS: Decouple USB events from the USB interrupt handler

[tzarc]

Description

Struck a bit of an issue when using backlight and an external EEPROM -- the backlight handler issues a read which can eventually trigger a thread suspension.... the problem is, this is done from within an interrupt handler:

Attempting a thread suspension triggers an unhandled exception and the MCU subsequently locks up.
This PR pushes the event handling to the main keyboard loop.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[tzarc]

This was triggered on the Djinn prototype -- whenever backlight was enabled with the SPI EEPROM at the same time, bootup of the keyboard would lock up every single time.
Disabling backlight, or using the transient EEPROM would allow the keyboard to boot.

[tzarc]

Testing requested: suspend handling.

[tzarc]

Looks as if this isn't happy if it's running on slave side of a split.
Never mind, hadn't updated both sides at the same time.

Added some cleanup and initialisation code as well, working fine on both sides of the split.