net: dsa: b53: do not configure bcm63xx's IMP port interface

net: dsa: b53: do not configure bcm63xx's IMP port interface

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@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20250602193953.1010487-4-jonas.gorski@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>