net: stmmac: fix enabling socfpga's ptp_ref_clock

There are 2 registers to write to enable a ptp ref clock coming from the
fpga.
One that enables the usage of the clock from the fpga for emac0 and emac1
as a ptp ref clock, and the other to allow signals from the fpga to reach
emac0 and emac1.
Currently, if the dwmac-socfpga has phymode set to PHY_INTERFACE_MODE_MII,
PHY_INTERFACE_MODE_GMII, or PHY_INTERFACE_MODE_SGMII, both registers will
be written and the ptp ref clock will be set as coming from the fpga.
Separate the 2 register writes to only enable signals from the fpga to
reach emac0 or emac1 when ptp ref clock is not coming from the fpga.

Signed-off-by: Julien Beraud <julien.beraud@orolia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>