Kernel/USB: Implement test transfer

We can now test a _very_ basic transaction via `do_debug_transfer()`.
This function merely attaches some TDs to the LSCTRL queue head
and points some input and output buffers. We then sense an interrupt
with USBSTS value of 1, meaning Interrupt On Completion
(of the transaction). At this point, the input buffer is filled with
some data.

Kernel/init.cpp

@@ -247,7 +247,6 @@ void init_stage2(void*)
     }
 
     USB::UHCIController::detect();
-    USB::UHCIController::the().spawn_port_proc();
 
     E1000NetworkAdapter::detect();
     RTL8139NetworkAdapter::detect();