Function mv88e6xxx_serdes_pcs_get_state() currently does not report link
up if AN is enabled, Link bit is set, but Speed and Duplex Resolved bit
is not set, which testing shows is the case for when auto-negotiation
was bypassed (we have AN enabled but link partner does not).

An example of such link partner is Marvell 88X3310 PHY, when put into
the mode where host interface changes between 10gbase-r, 5gbase-r,
2500base-x and sgmii according to copper speed. The 88X3310 does not
enable AN in 2500base-x, and so SerDes on mv88e6xxx currently does not
link with it.

Fix this.

Fixes: a5a6858b ("net: dsa: mv88e6xxx: extend phylink to Serdes PHYs")
Signed-off-by: NMarek Behún <kabel@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>