Staging: comedi: adv_pci1710: Remove unneeded chanlist_len check

For AI command testing, the driver does not need to range-check
the length of the channel list as the comedi core has already
checked it.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>