net: dsa: b53: Fix IMP port setup on BCM5301x

[ Upstream commit 63f8428b ]

Broadcom's b53 switches have one IMP (Inband Management Port) that needs
to be programmed using its own designed register. IMP port may be
different than CPU port - especially on devices with multiple CPU ports.

For that reason it's required to explicitly note IMP port index and
check for it when choosing a register to use.

This commit fixes BCM5301x support. Those switches use CPU port 5 while
their IMP port is 8. Before this patch b53 was trying to program port 5
with B53_PORT_OVERRIDE_CTRL instead of B53_GMII_PORT_OVERRIDE_CTRL(5).

It may be possible to also replace "cpu_port" usages with
dsa_is_cpu_port() but that is out of the scope of thix BCM5301x fix.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>