Use the device unit number for naming the ifnet interface in mlx5en(4).

Currently the ifnet interface is named mceX, where X is a monotonically
incremented value. If the device is reset due to a fatal error, then the
interface name will change. Using the device unit number will keep the
naming consistent across the reset logic.

Submitted by: Matthew Finlay <matt@mellanox.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies