net: phy: DP83TC811: Fix SGMII enable/disable

If SGMII was selected in the DT then the device should
write the SGMII enable bit.

If SGMII is not selected in the DT then the SGMII bit
should be disabled.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>