net: stmmac: ingenic: simplify x2000 mac_set_mode()

net: stmmac: ingenic: simplify x2000 mac_set_mode()

As per the previous commit, we have validated that the phy_intf_sel
value is one that is permissible for this SoC, so there is no need to
handle invalid PHY interface modes. We can also apply the other
configuration based upon the phy_intf_sel value rather than the
PHY interface mode.

Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1vHHqO-0000000Djrb-0DPN@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>