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
Differential Revision: https://reviews.freebsd.org/D57242

(cherry picked from commit b1f3726f7a677ae230135d19491ce97f2866e7c2)