Default state: What to do on reset

Last time, we left one bit hanging. We said we don’t know what to do on
reset. Now we know what to expect, so we can think about it.

* We need to be able to receive Reset, Suspend and Resume events. The
  `EFSR` bit in `IEN` gives us that but we will need to implement
  reaction to the `USUS` flag.
* We also need to receive USB requests. These come as data received via
  endpoint 0, so we need to enable `EF` in `IEN` and `URXIE0` in `UIE`;
  when data comes in, we get a USB interrupt with the `URXD0` flag set.
* We will also have to transmit data in response to the requests. The
  MG84FL54 has a limited FIFO buffer through which we can transmit, so
  we need to know when the transmission is complete and write more data
  into the FIFO. This is controlled by the `UTXIE0` flag in `UIE`; we
  will receive a USB interrupt with `UTXD0` set.