net: bcmgenet: Refactor register access in bcmgenet_mii_config

The register access in bcmgenet_mii_config() is a little bit opaque and
not easy to extend. In preparation for the missing RGMII PHY modes
move all the phy name assignments into the switch statement and the
register access to the end of the function. This make the code easier
to read and extend.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: David S. Miller <davem@davemloft.net>