samples/subsys/zbus/benchmark: fix counter synchronization issue

Current implementation has obvious synchronization issue with
global counter variable which is updated (RMW) in multiple threads
without any locking.

Replace regular variable by atomic type. As the maximum possible
value is much less than INT32_MAX it's OK to replace the original
uint64_t by (32-bit) atomic_t.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>