pm: device_runtime: Optimize pm_device_runtime_usage

There is no point in using lock or semaphore to read current
usage counter as it may change after unlocking or giving
back the semaphore. Value can only be trusted in the controlled
environment (e.g. test).

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>