net/mlx5: Fix fw tracer first block check

While handling new traces, to verify it is not the first block being
written, last_timestamp is checked. But instead of checking it is non
zero it is verified to be zero. Fix to verify last_timestamp is not
zero.

Fixes: c71ad41c ("net/mlx5: FW tracer, events handling")
Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Feras Daoud <ferasda@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>