net/wifi-firmware-mt76-kmod: update firmware files

With the removal of the "USE=kmod" and only building packages with
plain firmware files, our version went backwards. Updating the
firmware will fix this. For mt76 we can just do that as we
do not yet ship the drivers officially.

Change to a script to extract the firmware files from WHENCE to get
them all.

Sponsored by: The FreeBSD Foundation
Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D54201