rtwn: add support for AES-GCM-128 in the supported software cipher set

I'm testing this with rtwn(4) NICs right now, so enable it here.

Locally tested:

• RTL8812AU / RTL8821AU, STA mode (with some wpa_supplicant.conf shenanigans to force GCMP as the pairwise key to an OpenWRT AP configured to use GCMP but without requiring MFP.)

Differential Revision: https://reviews.freebsd.org/D49190
Reviewed by: bz