net/mlx5_core: Fix static checker warnings around system guid query flow

Fix static checker warnings in the flow of system guid query.

Fixes: 707c4602

 ('net/mlx5_core: Add new query HCA vport commands')
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>