net: phy: marvell: More hidden page changes refactored

EXT_ADDR_PAGE is the same meaning as MII_MARVELL_PHY_PAGE, i.e. change
page. Replace it will calls to the helpers.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>