staging: comedi: adq12b: remove devpriv macro

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

Also, remove the kfree in the detach. The comedi core handles
the kfree of dev->private.

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>