PM / tools: cpupower: drop negativity check on unsigned value

Link: https://bugzilla.kernel.org/show_bug.cgi?id=80621


Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Andrey Utkin <andrey.krieger.utkin@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>