staging: comedi: amplc_pci230: rename 'todo' variable

This patch renames the very generic variable name 'todo' to
nsamples.

Signed-off-by: Hari Prasath Gujulan Elango <hgujulan@visteon.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>