net: phy: dp83867: increase SGMII autoneg timer duration

After reset SGMII Autoneg timer is set to 2us (bits 6 and 5 are 01).
That is not enough to finalize autonegatiation on some devices.
Increase this timer duration to maximum supported 16ms.

Signed-off-by: Max Uvarov <muvarov@gmail.com>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>