staging: comedi: me_daq: tidy up private data

For aesthetics, rename the local variable used to access dev->private.
In comedi drivers this variable is typically named 'devpriv'.

For aesthetics, rename the private data members used to mirror the
write-only register to, slightly, shorter names.

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