Revise cnmac(4)'s address filter setup

Revise cnmac(4)'s address filter setup

Set up the DMAC filter in one go instead of doing it separately for
unicast and multicast DMACs. This attempts to make the code a little
more readable. The setup should now run a bit faster as well because
it now does fewer register accesses.

Tested on CN5020, CN6120 and CN7130.