net/mlx5: Avoid dereferencing uninitialized pointer

In NETDEV_CHANGEUPPER event the upper_info field is valid
only when linking is true. Otherwise it should be ignored.

Fixes: 7907f23a (net/mlx5: Implement RoCE LAG feature)
Signed-off-by: Talat Batheesh <talatb@mellanox.com>
Reviewed-by: Aviv Heller <avivh@mellanox.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>