staging: comedi: dt3000: tidy up digital i/o subdevice init

Remove the unnecessary 'len_chanlist' initialization. This member is
only used by subdevices that support async commands.

For aesthetics, reorder the initialization a bit.

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>