LinuxKPI: 802.11: fix field order in ieee80211_key_conf

When adding the new field link_id to struct ieee80211_key_conf, it
was erroneously placed at the end of the struct; the zero-length
(variable sized) array for the key always needs to stay last.
Resort fields and add hopefully helpful comment to avoid the problem
in the future.

Approved by: re (cperciva)
Fixes: adff403fe7a87
Reviewed by: cc
Differential Revision: https://reviews.freebsd.org/D43635

(cherry picked from commit 828ce22f079df77b3bf52c4cf296959ea66bdc11)
(cherry picked from commit 1cf1f094680fe3bfd9836800f83ed0b79e180340)