mlx5: Fix parameter type of health_handler_t

This deals with the sparse warning:

    drivers/net/ethernet/mellanox/mlx5/core/health.c:94:54: warning: incorrect type in argument 2 (different address spaces)
    drivers/net/ethernet/mellanox/mlx5/core/health.c:94:54:    expected void *buf
    drivers/net/ethernet/mellanox/mlx5/core/health.c:94:54:    got struct health_buffer [noderef] <asn:2>*health

Signed-off-by: Roland Dreier <roland@purestorage.com>