staging: comedi: amplc_pc236: remove 'model' member

The `model` member of `struct pc236_board` is no longer used since the
code to remove manual configuration of PCI devices was removed.  Get rid
of it.

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