net: bcmgenet: Don't set ID_MODE_DIS when not using RGMII

As per Linux's driver, ID_MODE_DIS is only set when the PHY interface is
RGMII. Don't enable it for the rest of setups.

This has been seen to misconfigure RPi4's PHY when booting Linux.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>