net: bgmac: improve handling PHY

1. Use info from DT if available

It allows describing for example a fixed link. It's more accurate than
just guessing there may be one (depending on a chipset).

2. Verify PHY ID before trying to connect PHY

PHY addr 0x1e (30) is special in Broadcom routers and means a switch
connected as MDIO devices instead of a real PHY. Don't try connecting to
it.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>