net: macb: poll for fixed link state in SGMII mode

When using a fixed-link configuration with GEM in SGMII mode, such as
for a chip-to-chip interconnect, the link state was always showing as
established regardless of the actual connectivity state. We can monitor
the pcs_link_state bit in the Network Status register to determine
whether the PCS link state is actually up.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Signed-off-by: David S. Miller <davem@davemloft.net>