net: stmmac: mediatek: simplify set_interface() methods

net: stmmac: mediatek: simplify set_interface() methods

Use the phy_intf_sel field value when deciding what other options to
apply for the configuration register.

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/E1vIjU4-0000000DqtV-3qsX@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>