staging: comedi: aio_aio12_8: prefer using the BIT macro

As suggested by checkpatch.pl, use the BIT macro to define the
register bits.

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>