logging: refactor log_filter_set to set existing log level

Modified log_filter_set function to limit level if requested
level is not compiled in. Additionally, extended function to
return actually set level. Removed redundant code from log_cmds.

Change fixes shell log backend initialization which was setting
log levels without taking into account compiled in limits.

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