logging: log_output: Fix for Coverity issue 316014

Log level is stored on 3 bits thus in theory it can be set to 7
and yet accepted levels are up to 4 thus Coverity complains.
Adding assert that prevents use of levels above 4.

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