The IMP port is not a valid RGMII interface, but hard wired to internal,
so we shouldn't touch the undefined register B53_RGMII_CTRL_IMP.
While this does not seem to have any side effects, let's not touch it at
all, so limit RGMII configuration on bcm63xx to the actual RGMII ports.
Fixes: ce3bf94871f7 ("net: dsa: b53: add support for BCM63xx RGMIIs")
Signed-off-by: Jonas Gorski <jonas.gorski@xxxxxxxxx>