USB4 acknowledge hotplug events

[obiwac]

Summary

When getting a hotplug event from the hardware, transmit a hotplug acknowledgment packet. This is needed even if we don't do anything with the hotplug events, as the hardware will continuously "spam" us with hotplug events otherwise.

Test Plan

Tested on AMD Pink Sardine controller (1022:1668/9), which is host interface version 1.0.

[obiwac]

I want to factor out a notify function first.

[imp]

I'll take a look once you're done with the planned changes you've flagged.

[obiwac]

Fixes