Prevent ieee80211_get_txkey() from returning the integrity group temporal

Prevent ieee80211_get_txkey() from returning the integrity group temporal
key (IGTK) if a node doesn't have management frame protection (MFP) enabled.
The IGTK is not initialized if MFP is disabled, so using it triggers this
panic in ieee80211_encrypt(): panic("invalid key cipher 0x%x", k->k_cipher)

(As far as I can tell, at present, MFP is never enabled.)

Problem reported and fix tested by tj@ on athn(4) hostap