net: phy: Utilize phy_interface_is_rgmii

Update all open-coded tests for all 4 PHY_INTERFACE_MODE_RGMII* values
to use the newly introduced helper: phy_interface_is_rgmii.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>