net: ethernet: bgmac: unify code of the same family

BCM471X and BCM535X are of the same family (from what I can derive from
internal documents).  Group them into the case statement together, which
results in more code reuse.

Also, use existing helper variables to make the code a little more
readable too.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>