staging: comedi: amplc_dio200_common: use comedi_cmd pointer

Use the local variable to access the comedi_cmd as a pointer
in dio200_read_scan_intr() instead of getting to it from the
comedi_subdevice pointer.

Remove the unnecessary local variable 'len', this is just the
cmd->chanlist_len.

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>