staging: comedi: me4000: fix aref test in ai_write_chanlist()

The 'aref' (CR_AREF) values are all AREF_* defines. The SDF_* defines are
subdevice flags. Fix the test in this function.

Reported by: coverity
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>