net: stmmac: meson8b: use stmmac_get_phy_intf_sel()

net: stmmac: meson8b: 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.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vIT6b-0000000DpPX-1LQ0@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>