mv643xx_eth: remove force_phy_addr field

Currently, there are two different fields in the
mv643xx_eth_platform_data struct that together describe the PHY
address -- one field (phy_addr) has the address of the PHY, but if
that address is zero, a second field (force_phy_addr) needs to be
set to distinguish the actual address zero from a zero due to not
having filled in the PHY address explicitly (which should mean
'use the default PHY address').

If we are a bit smarter about the encoding of the phy_addr field,
we can avoid the need for a second field -- this patch does that.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>