Staging: comedi: addi-data: Cleanup redundant tests on unsigned

The variables are unsigned so the test `>= 0' is always true,
In these cases the other part of the test catch wrapped values.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>