staging: comedi: adv_pci1710: only init async command members when needed

The 'len_chanlist' and 'cancel' members of the comedi_subdevice are
only used with async command support. Only initialize them if the irq
was sucessfully requested.

Also, only init the dev->read_subdev if we have the irq.

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>