fwget: update MediaTek firmware listings to match ports

Update fwget(8) to download wifi-firmware-mt76-kmod-mt7921, and
wifi-firmware-mt76-kmod-mt7925 firmware packages instead of the
no longer available mt792x version.
Add another PCI vendor to recognize ITTIM IDs for mt7921-based
MediaTek cards.

(bz reduced the license in the ittim file to an SPDX tag and updated
the commit message, given this is only half the work from the review)

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D57242