[xhci] Consume event TRB before reporting completion to USB core

Reporting a completion via usb_complete() will pass control outside
the scope of xhci.c, and could potentially result in a further call to
xhci_event_poll() before returning from usb_complete().  Since we
currently update the event consumer counter only after calling
usb_complete(), this can result in duplicate completions and
consequent corruption of the submission TRB ring structures.

Fix by updating the event ring consumer counter before passing control
to usb_complete().

Reported-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Tested-by: Andreas Hammarskjöld <junior@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/usb/xhci.c

 			   ( event->cons >> shift ) ) & XHCI_TRB_C ) )
 			break;
 
+		/* Consume this TRB */
+		event->cons++;
+
 		/* Handle TRB */
 		type = ( trb->common.type & XHCI_TRB_TYPE_MASK );
 		switch ( type ) {
 
 		default:
 			DBGC ( xhci, "XHCI %s unrecognised event %#x\n:",
-			       xhci->name, event->cons );
+			       xhci->name, ( event->cons - 1 ) );
 			DBGC_HDA ( xhci, virt_to_phys ( trb ),
 				   trb, sizeof ( *trb ) );
 			break;
 		}
-
-		/* Consume this TRB */
-		event->cons++;
 	}
 
 	/* Update dequeue pointer if applicable */