if_enc(4): Use new KPI to create enc interface

This driver allows only exactly one instance to be created. Clone
creating additional interfaces, e.g. ifconfig enc1 create, will get
error EEXIST which is somewhat confusing.

Convert to new KPI for less confusing error ENOSPC.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45758