Use static device naming instead of dynamic one in mlx5ib.

When resetting mlx5core instances it can happen that the order of attach and
detach for mlx5ib instances is changed. Take the unit number for mlx5_%d from
the parent PCI device, similarly to what is done in mlx5en(4), so that there
is a direct relationship between mce<N> and mlx5_<N>.

MFC after: 1 week
Sponsored by: Mellanox Technologies