net80211: Fix IEEE80211_KEY_UNDEFINED() in ieee80211_node_delucastkey()

The adjustment of the check in 9763fec11b83 inverted the logic. Fix this.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Fixes: 9763fec11b83
Reported by: KUROSAWA Takahiro (takahiro.kurosawa gmail.com) in D48980
Reviewed by: KUROSAWA Takahiro (takahiro.kurosawa gmail.com), adrian
Differential Revision: https://reviews.freebsd.org/D49234

(cherry picked from commit 27bf5c405bf2eb69392e45c06605defc78882612)