net/mlx5_core: Check the return value of mlx5_command_exec()

mlx5_cmd_exec() might fail - need to check return value.

Signed-off-by: Haggai Abramovsky <hagaya@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>