net: dsa: b53: Drop BCM5301x workaround for a wrong CPU/IMP port

On BCM5301x port 8 requires a fixed link when used.

Years ago when b53 was an OpenWrt downstream driver (with configuration
based on sometimes bugged NVRAM) there was a need for a fixup. In case
of forcing fixed link for (incorrectly specified) port 5 the code had to
actually setup port 8 link.

For upstream b53 driver with setup based on DT there is no need for that
workaround. In DT we have and require correct ports setup.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>