logging: Ensure constant side effects of log API

When log API is used it should give the same side effect when enabled
or disabled e.g. LOG_INF(%d,cnt++) should always increment cnt.
Before this change cnt would be incremented twice when log enabled
and never incremented when log was disabled.

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