net/mlx5: Fix fw reporter diagnose output

Restore fw reporter diagnose to print the syndrome even if it is zero.
Following the cited commit, in this case (syndrome == 0) command returns no
output at all.

This fix restores command output in case syndrome is cleared:
$ devlink health diagnose pci/0000:82:00.0 reporter fw
    Syndrome: 0

Fixes: d17f98bf

 ("net/mlx5: devlink health: use retained error fmsg API")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>