at803x: fix suspend/resume for SGMII link

When operating the at803x in SGMII mode, resuming the chip
from power down brings up the copper-side link but leaves
the SGMII link in unconnected state (tested with at8031
attached to gianfar). In effect, this caused a permanent
link loss once the related interface was put down.

This patch ensures that power down handling in supspend()
and resume() is also applied to the SGMII link.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: David S. Miller <davem@davemloft.net>