usb: storage: shuttle_usbat: fix discs being detected twice

commit df602c2d2358f02c6e49cffc5b49b9daa16db033 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>