rtic-usb-cdc-echo example generates warnings

[sre]

With latest Rust compiler, the USB example generates this warning:

    |             USB_BUS.replace(UsbBus::new(usb, &mut EP_MEMORY));
    |                                              ^^^^^^^^^^^^^^ mutable reference of mutable static
    |
    = note: for more information, see issue #114447 <https://github.com/rust-lang/rust/issues/114447>
    = note: reference of mutable static is a hard error from 2024 edition
    = note: mutable statics can be written to by multiple threads: aliasing violations or data races will cause undefined behavior
help: mutable references are dangerous since if there's any other pointer or reference used for that static while the reference lives, that's UB; use `addr_of_mut!` instead to create a raw pointer
    |
    |             USB_BUS.replace(UsbBus::new(usb, addr_of_mut!(EP_MEMORY)));
    |                                              ~~~~~~~~~~~~~~~~~~~~~~~

warning: mutable reference of mutable static is discouraged