net: bcmgenet: Clear ID_MODE_DIS in EXT_RGMII_OOB_CTRL when not needed

Outdated Raspberry Pi 4 firmware might configure the external PHY as
rgmii although the kernel currently sets it as rgmii-rxid. This makes
connections unreliable as ID_MODE_DIS is left enabled. To avoid this,
explicitly clear that bit whenever we don't need it.

Fixes: da388022 ("net: bcmgenet: Add RGMII_RXID support")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>