staging: comedi: adq12b: remove need for some local variables

The 'unipolar' and 'differential' local variables are not needed
in the adb12b_attach() function. Just use the cached private
data variables.

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