Finally plug the public xfer leak #1 in our USB stack.

Finally plug the public xfer leak #1 in our USB stack.

Every call to usbd_abort_pipe() on an interrupt pipe would simply
reset the intrxfer pointer, which would prevent usbd_close_pipe()
to free it.  Since we abort pipes in a lot of situations: when a
device is detached, when a USB-to-serial adapter is closed, when
an error occurs, when the machine is suspended, etc, this would
result in hundreds of leaked xfers in most of my machines.

xhci(4) is not affected, but you can't enable it right now since
the stack is not ready :)

While here put a KASSERT() to make sure drivers are only calling
the interrupt abort method for intrxfer, if that's not the case,
please let met know.