logging: Ensure atomic update of log filter slot in LOG_FILTER_SLOT_SET

Replaced the read-modify-write sequence with a single read and write
operation, preventing the intermediate value is wrongly used to filter
out logs of another thread with higher priority that preempts the current
thread.

Signed-off-by: Dong Wang <dong.d.wang@intel.com>
(cherry picked from commit 872f3636)