staging: comedi: aio_aio12_8: remove devpriv macro

This macro relies on a local variable having a specific name. Remove
it.

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>