log: fix a harmless bug

Z_LOG2() has a typo, confusing & with &&. The result is the same but
using && is more logical in that case and it eliminates a flood of

warning: dubious: x & !y

static analyser warnings. Also compare to 0 explicitly to fix a
coding style violation.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>