sfc: QT2025C: Work around PHY bug

If we see the PHY remaining stuck in a link-down state due to PCS being
down while PMA/PMD is up, we briefly switch to PMA/PMD loopback and back,
which usually unsticks it.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>