ether_ifattach() does not touch if_type. This change will ensure that the
event listener, e.g. ifnet_arrival_event, see the right interface
type. Also other threads will see consistent interface type.
As a nice effect, this change eliminates the memory allocation for
if_hw_addr, since vlan(4) interfaces do not support setting or
retrieving the hardware MAC address yet [1].
[1] ddae57504b79 Persistently store NIC's hardware MAC address, and add a way to retrive it
MFC after: 1 week