usb: display power supply overload notifications

[hnez]

Add warning on the LCD …

… and web interface if the LXA TAC' USB ports are overloaded.

[hnez]

I've implemented the overcurrent warning level constants as @KarlK90 suggested and fixed some merge conflicts while rebasing on the current main branch,
Please re-approve and merge at will.

[hnez]

Changes introduced by #27 necessitated some changes with regard to error handling of ADC readings, which I have just pushed:

tacd/src/usb_hub.rs

Lines 291 to 304 in b38944c

	spawn(async move {
	loop {
	let overloaded_port = OverloadedPort::from_currents(
	total.get().map(|m| m.value).unwrap_or(0.0),
	port1.get().map(|m| m.value).unwrap_or(0.0),
	port2.get().map(|m| m.value).unwrap_or(0.0),
	port3.get().map(|m| m.value).unwrap_or(0.0),
	);
	overload_task.set_if_changed(overloaded_port);
	sleep(POLL_INTERVAL).await;
	}
	});

In addition to that I've also implemented @KarlK90's suggestion for inlining the let overload_* bindings into the if … else if … chain.

[hnez]

Hi @KarlK90 can I get a new approval? The previous one was invalidated by required changes in a rebase.

[KarlK90]

@hnez done!