b44: check register instead of PHY address to detect external PHY

The Ethernet core supported by b44 supports an internal PHY integrated
into the mac core, which is supported by the b44 driver and an external
PHY to which the mac core is connected. This external PHY could be a
switch connected through MII, which is often the case when this core is
used on home routers. The usage of an external PHY was assumed when the
PHY address 30 was used and an internal PHY was assumed when the PHY
address was different. To verify that b44_phy_reset() was called and
checked if it worked, otherwise PHY address 30 was assumed, an external
PHY. It is better to check the register which says which PHY is
connected to the MAC instead of checking the PHY address.
The interface to an external PHY was only activated when this register
was set.

This also changes B44_FLAG_INTERNAL_PHY to B44_FLAG_EXTERNAL_PHY, it is
easier to check.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>