staging: comedi: pcmda12: tidy up subdevice init

For aesthetic reasons. add some whitespace to the subdevice init
and reorder it a bit.

Remove the 's->private = NULL', it was kzalloc'ed by the core and
will already be NULL.

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>