staging: comedi: addi_apci_3xxx: fix ai subdevice 'len_chanlist'

Due to a driver limitation, the (*do_cmdtest) for the analog input subdevice
overrides the cmd->chanlist_len to force single channel chanlists. But, the
cmd->scan_end_arg was already validated before this happens. Instead of
fixing the (*do_cmdtest) just set the subdevice len_chanlist to 1 to limit
the chanlist size. Move the comment also.

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>