net80211: use IEEE80211_KEY_UNDEFINED() insetad of checking they keyix

For ieee80211_node_delucastkey() rather than checking the keyix to be
IEEE80211_KEYIX_NONE use the IEEE80211_KEY_UNDEFINED() macro (which
chekes the wk_cipher to be 'none').

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48980

(cherry picked from commit 9763fec11b83a7bfc2d2ba0681830d186a3f6a6e)