net: don't overwrite net_if name after iface is initialized

When interface names are enabled, a default name is applied
after initialization. Unintuitively, this overwrites any name that is
set during the net_if init.

This change sets a default name first, and then allows net_if
init to overwrite that default name if it chooses to.

Signed-off-by: Noah Luskey <noah@silvertree.io>
Signed-off-by: Noah Luskey <LuskeyNoah@gmail.com>