net/mlx5: cmdif, cmd_check refactoring

Do not mangle the command outbox in the internal low level cmd_exec and
cmd_invoke functions.

Instead return a proper unique error code and move the driver error
checking to be at a higher level in mlx5_cmd_exec().

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>