net: stmmac: loongson1: use stmmac_get_phy_intf_sel()

net: stmmac: loongson1: 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.

Note that this will allow GMII as well as MII as the phy_intf_sel
value is the same for both.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/E1vIjTp-0000000DqtC-2DmI@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>