net: stmmac: dwmac-qcom-ethqos: Use of_get_phy_mode() over device_get_phy_mode()

Since this driver only uses devicetree, let's move over to
of_get_phy_mode(). That API has an explicit error return and produces a
phy_interface_t directly instead of an int when representing the phy
mode.

Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>