Blackfin: bfin_mac: cleanup MII/PHY functions

Cleanup and rewrite the MII/PHY related functions so that we can reuse the
existing common linux/miiphy.h code and hook into the `mii` command.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>