setup-disk: fix missing package error for b43 fw

select_firmware_pkgs creates a list of addional linux-firmware-<module> to be installed based on /sys/module/<module> during 'sys' installation. Exclude missing b43 if it's found there.

I am not sure if this should include b43legacy as well https://wireless.wiki.kernel.org/en/users/Drivers/b43#Supported_devices

Tested on a device with Broadcom BCM4313 adapter

ref #10530 (closed)