staging: comedi: dt9812: do not allow attaching to unsupported devices

The DT9812 is part of the ECONseries of USB Data Acquisition Modules.
This driver only supports the DT9812-10V and DT9812-2.5V versions of
the module. Do not attach to the device if an unsupported module is
detected.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>