hw/net/cadence_gem: perform PHY access on write only

hw/net/cadence_gem: perform PHY access on write only

The MDIO access is done only on a write to the PHYMNTNC register. A
subsequent read is used to retrieve the result but does not trigger an
MDIO access by itself.

Refactor the PHY access logic to perform all accesses (MDIO reads and
writes) at PHYMNTNC write time.

Signed-off-by: Luc Michel <luc.michel@amd.com>
Reviewed-by: sai.pavan.boddu@amd.com
Message-id: 20231017194422.4124691-11-luc.michel@amd.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>