net80211: add AES-GCM to the hostap logic

This is currently an untested diff set for implementing the
AES-GCM negotiation in hostap mode.

• Decode the AES-GCM-128 cipher in the RSN field;
• Add AES-GCM as the first cipher to check when deciding the unicast cipher type;
• Refactor out the "can we do HT A-MPDU + this cipher" check for the unicast cipher; and
• .. add AES-GCM-128 to the allowable ciphers.

I haven't tested this yet to make sure I haven't broken the hostapd
path, nor that it actually DOES negotiate AES-GCM-128.

Differential Revision: https://reviews.freebsd.org/D49189