net: bcmgenet: power on MII block for all MII modes

The RGMII block is currently only powered on when using RGMII or
RGMII_NO_ID, which is not correct when using the GENET interface in MII
or Reverse MII modes. We always need to power on the RGMII interface for
this block to properly work, regardless of the MII mode in which we
operate.

Fixes: aa09677c ("net: bcmgenet: add MDIO routines")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>