net/ng_mikrotik_eoip: Fix build after recent changes in main.

src commit 6d5f002ed1187547b0b40f8b46a3db38bf89c6dc changed the API of
NG_NODE_FOREACH_HOOK to remove a mostly unused parameter. Use a
structure as the callback argument to save the returned hook.

PR: 266392
Differential Revision: https://reviews.freebsd.org/D36545