vdpa/mlx5: Use eth_broadcast_addr() to assign broadcast address

Using eth_broadcast_addr() to assign broadcast address instead
of memset().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xu Qiang <xuqiang36@huawei.com>
Message-Id: <20220704021405.64545-1-xuqiang36@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>