net: ethernet: set default assignment identifier to NET_NAME_ENUM

As noted in the original commit 685343fc

 ("net: add
name_assign_type netdev attribute")

  ... when the kernel has given the interface a name using global
  device enumeration based on order of discovery (ethX, wlanY, etc)
  ... are labelled NET_NAME_ENUM.

That describes this case, so set the default for the devices here to
NET_NAME_ENUM.  Current popular network setup tools like systemd use
this only to warn if you're setting static settings on interfaces that
might change, so it is expected this only leads to better user
information, but not changing of interfaces, etc.

Signed-off-by: Ian Wienand <iwienand@redhat.com>
Link: https://lore.kernel.org/r/20220406093635.1601506-1-iwienand@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>