Restructure phy_write_mmd and phy_read_mmd to implement generic helper
for direct mdiobus access for mmd and use these helper for phydev user.

This is needed in preparation of PHY package API that requires generic
access to the mdiobus and are deatched from phydev struct but instead
access them based on PHY package base_addr and offsets.
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>