PM / Runtime: dev_pm_info.runtime_error is signed

dev_pm_info.runtime_error has always been a signed int, to store a signed
error code. Correct the documentation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>