net: stmmac: sti: use stmmac_get_phy_intf_sel()

net: stmmac: sti: 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 when an unsupported interface mode is used, the array would
decode this to PHY_INTF_SEL_GMII_MII, so preserve this behaviour.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vGy5j-0000000DhQh-2e0x@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>