logging: Minor cleanup in logging helper macros

Change logging level definitions to just numbers instead of
numbers with unsigned indicator (e.g. 1 instead of 1U). Levels
from Kconfig comes as just numbers and often levels are used
for IS_ENABLED-type of macros where values are concatenated
into tokens and it actually makes the difference if value is
1 or 1U. It allows minor cleanup in the internal macros.

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