logging: Fix case when LOG_LEVEL is 0

Setting LOG_LEVEL to 0 was not covered. It resulted in the
logging misbehavior when module logging level was set using
LOG_LEVEL define method and it was set to 0. Instead of
filtering all levels it was applying the default filtering.

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