net: stmmac: socfgpa: Ensure emac bit set in sys manager for MII/GMII/SGMII.

When using MII/GMII/SGMII in the Altera SoC, the phy needs to be
wired through the FPGA. To ensure correct behavior, the appropriate
bit in the System Manager FPGA Interface Group register needs to be
set.

Signed-off-by: Stephan Gatzka <stephan.gatzka@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>