STM32: panic when initializing USB ULPI

[mattico]

The panic occurs here:

embassy/embassy-stm32/src/usb/mod.rs

Line 15 in 51d5530

let freq = T::frequency();

I think this is because the ULPI PHY provides the 48MHz clock, so the internal 48MHz clock isn't enabled. It may be due to some other initialization registers which need to be set for ULPI.

I will likely investigate tomorrow.

[mattico]

I forgot to configure the usb clock mux after I updated embassy.

It seems obvious now, but it was difficult to root cause because the panic message's line info would often just point to fmt.rs or sometimes _generated::{impl#140}::frequency() which wasn't super helpful. It would be nice if the panic message would say "configure config.rcc.mux.usbsel" rather than just "unreachable".