ar8216: Create helpers mii_read32 / mii_write32 for 32 bit MII ops

Create helpers mii_read32 / mii_write32 for 32 bit MII ops.
Rename r3 variable to page in ar8xxx_mii_write to make it consistent
with the other ar8xxx_mii_xxxx functions.

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

SVN-Revision: 43741