net: stmmac: intel: convert speed_mode_2500() to get_interfaces()

net: stmmac: intel: convert speed_mode_2500() to get_interfaces()

TGL platforms support either SGMII or 2500BASE-X, which is determined
by reading a SERDES register.

Thus, plat->phy_interface (and phylink's supported_interfaces) depend
on this. Use the new .get_interfaces() method to set both
plat->phy_interface and the supported_interfaces bitmap.

This removes the only user of the .speed_mode_2500() method.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1uASLx-0021Qs-Uz@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>