usb: storage: shuttle_usbat: fix discs being detected twice

Even if the USB-to-ATAPI converter supported multiple LUNs, this
driver would always detect the same physical device or media because
it doesn't use srb->device->lun in any way.
Tested with an Hewlett-Packard CD-Writer Plus 8200e.

Signed-off-by: Daniele Forsi <dforsi@gmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>