LinuxKPI: add three skeleton functions to etherdevice.h

We will see how much we will need them and how to fill them at that
point. They are left with a pr_debug("TODO") for now.

Approved by: re (cperciva)
Differential Revision: https://reviews.freebsd.org/D53204

(cherry picked from commit cc792250acadd29e80a69501ad326d1340c0c1c6)
(cherry picked from commit 805038b70a23f3f488ecba6e103f2aa1f9dad63c)