I found a bug in the code making the kinect motor not tilt downward. To fix this change this line in the setTiltAngle function of the program:
From: const int ret = libusb_control_transfer(dev, 0x40, 0x31, (uint16_t)angle, 0x0, empty, 0x0, 0);
To: const int ret = libusb_control_transfer(dev, 0x40, 0x31, (int16_t)angle, 0x0, empty, 0x0, 0);