Refactor build for next upstream version and forky release cycle

1. Since new versions no longer need to be backported to bookworm, simplify installation by always installing firmware under /usr/lib/firmware.
2. Use dh_install and generated .install files for installation, to avoid an 'Argument list too long' error when updating to 20250808. (This is the essential change that I started off with.)
3. Having done that, simplify the build by using dh instead of the current recursive makefiles.
4. Finally, sort out the various clean rules in debian/rules, along the same lines as src:linux.