Fix variable name and error message for sideband testing

Actually the test only allows a range from [1,255], so let's name the
variable so.

Change-Id: Iecdb8149b83389c67e3cd2f64f4a654c175475be
Signed-off-by: Stefan Beller <sbeller@google.com>
Merge "Fix variable name and error message for sideband testing"