drivers: net: cpsw: fix RMII/RGMII mode when used with fixed-link PHY

Commit 1f71e8c9 ("drivers: net: cpsw: Add
support for fixed-link PHY") did not parse the "phy-mode" property in
the case of a fixed-link PHY, leaving slave_data->phy_if with its default
of PHY_INTERFACE_MODE_NA(0). This later gets passed to phy_connect() in
cpsw_slave_open(), and eventually to cpsw_phy_sel() where it hits a default
case that configures the MAC for MII mode.

The user visible symptom is that while kernel log messages seem to indicate
that the interface is set up, there is no network communication. Eventually
a watchdog error occurs:
    NETDEV WATCHDOG: eth0 (cpsw): transmit queue 0 timed out

Fixes: 1f71e8c9 ("drivers: net: cpsw: Add support for fixed-link PHY")
Signed-off-by: David Rivshin <drivshin@allworx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>