net/mlx5: add missing void argument to function mlx5_devlink_alloc

Function mlx5_devlink_alloc is missing a void argument, add it
to clean up the non-ANSI function declaration.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>