net: stmmac: starfive: use stmmac_get_phy_intf_sel()

net: stmmac: starfive: use stmmac_get_phy_intf_sel()

Use stmmac_get_phy_intf_sel() to decode the PHY interface mode to the
phy_intf_sel value, validate the result and use that to set the
control register to select the operating mode for the DWMAC core.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://patch.msgid.link/E1vIjUF-0000000Dqth-0gwD@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>