staging: comedi: adv_pci1723: remove comedi_device 'write_subdev' init

The comedi_device 'write_subdev' should only be initialized if the subdevice
supports async commands.

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>