net: phy: select copper mode when Marvel 88e1111 in SGMII

For the Marvel 88e1111 PHY only two SGMII modes are available, both
allowing only SGMII to copper mode (with or without clock). SGMII
to fiber mode is not supported. Make sure the fiber/copper registers
selector bits are cleared for selecting copper mode.

Signed-off-by: Madalin Bucur <madalin.bucur@freescale.com>
Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>