mt76: update script to to deal with 7921 and 7925 flavors

Update the mt76/zzz_fw_ports_fwget.sh script to set fwget to download
mt7921 and mt7925 rather than the these days non-existent mt792x flavor.

Sponsored by: The FreeBSD Foundation
MFC after: 30 days
Differential Revision: https://reviews.freebsd.org/D57242