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>

drivers/net/ethernet/mellanox/mlx5/core/diag/fw_tracer.c

@@ -718,7 +718,7 @@ static void mlx5_fw_tracer_handle_traces(struct work_struct *work)
 
 	while (block_timestamp > tracer->last_timestamp) {
 		/* Check block override if it's not the first block */
-		if (!tracer->last_timestamp) {
+		if (tracer->last_timestamp) {
 			u64 *ts_event;
 			/* To avoid block override be the HW in case of buffer
 			 * wraparound, the time stamp of the previous block