net: dsa: mt7530: add support for in-band link status

Read link status from SGMII PCS for in-band managed 2500Base-X and
1000Base-X connection on a MAC port of the MT7531. This is needed to
get the SFP cage working which is connected to SGMII interface of
port 5 of the MT7531 switch IC on the Bananapi BPi-R3 board.
While at it also handle an_complete for both the autoneg and the
non-autoneg codepath.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>