tools: Avoid showing return value of clock_gettime()

tools: Avoid showing return value of clock_gettime()

This value is either 0 for success or -1 for error. Coverity reports that
"ret" is passed to a parameter that cannot be negative, pointing to the
condition 'if (ret < 0)'.

Adjust it to just check for non-zero and avoid showing -1 in the error
message, which is pointless. Perhaps these changes will molify Coverity.

Reported-by: Coverity (CID: 312956)
Signed-off-by: Simon Glass <sjg@chromium.org>