net/mlx5: Check return status first when querying system_image_guid

When querying system_image_guid from firmware, we should check return
value first. The buffer content is valid only if query succeed.

Signed-off-by: Rongwei Liu <rongweil@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>