net: mvneta: Remember fixed link instead of PHY address in priv data

We don't need to remember PHY address anymore, because since using DM
MDIO for connecting PHY, the address is parsed by mdio-uclass from
the ofnode.

But the driver uses a special value of the address to signal fixed link
usage.

Drop phyaddr add fixed_link in driver private structure. This simplifies
code a little.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>