ar8216: Remove read/write/rmw member functions from ar8xxx_priv

Remove read/write/rmw member functions from ar8xxx_priv

There seems to be no real benefit of the ar8xxx_priv member functions
read/write/rmw as one implementation exists for each of them only.
Especially ar8xxx_mii_rmw is assigned to priv->rmw first and then
mapped to ar8xxx_rmw.
Rename the ar8xxx_mii_.. functions to ar8xxx_.. and use them directly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>

SVN-Revision: 43742