net/mlx5: Vport, Use 'kvfree()' for memory allocated by 'kvzalloc()'

When 'kvzalloc()' is used to allocate memory, 'kvfree()' must be used to
free it.

Fixes: 9efa7525 ("net/mlx5_core: Introduce access functions to query vport RoCE fields")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>