Fix for use after free.

Clear the device description to avoid use after free because the
bsddev is not destroyed when the mlx5en module is unloaded. Only when
the parent mlx5 module is unloaded the bsddev is destroyed. This fixes
a panic on listing sysctls which refer strings in the bsddev after the
mlx5en module has been unloaded.

Sponsored by: Mellanox Technologies
MFC after: 1 week