[xhci] Ring doorbell as part of endpoint reset

The endpoint may already have enqueued TRBs at the time that
xhci_endpoint_reset() is called.  Ring the doorbell to resume
processing these TRBs immediately, rather than waiting until the next
call to xhci_endpoint_message() or xhci_endpoint_stream().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/usb/xhci.c

 	if ( ( rc = xhci_set_tr_dequeue_pointer ( xhci, slot, endpoint ) ) != 0)
 		return rc;
 
+	/* Ring doorbell to resume processing */
+	xhci_doorbell ( &endpoint->ring );
+
 	DBGC ( xhci, "XHCI %p slot %d ctx %d reset\n",
 	       xhci, slot->id, endpoint->ctx );
 	return 0;