staging: comedi: aio_aio12_8: tidy up digital I/O subdevice init

For aesthetics, move and reword the comment to follow the format of
the other subdevices.

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>