net80211: cleanup keyidx argument of ieee80211_notify_michael_failure()

Instead of passing -1 to the function in ieee80211_crypto_demic(),
use the defined IEEE80211_KEYIX_NONE.
net80211 uses ieee80211_keyix not u_int as type for the key index
internally. Use that. This also helps when printing the -1 unsigned
as it stays within the expected numerical scope and makes the debug
message more readable.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D49368