snippets: nordic-log-stm: Do no enforce UART Asynchronous API

UART asynchronous API will ensure highest throughput but it is not
necessary as polling API is also supported as a fallback. Do not
enforce use of asynchronous API as there might be cases when polling
might be used (e.g. to reduce memory footprint).

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