staging: comedi: adv_pci1723: remove subdevice 'len_chanlist' initialization

The subdevice 'len_chanlist' is only used by subdevices that support async
commands. Remove the unnecessary initializations in this driver.

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>