net: dsa: b53: allow lower MTUs on BCM5325/5365

While BCM5325/5365 do not support jumbo frames, they do support slightly
oversized frames, so do not error out if requesting a supported MTU for
them.

Fixes: 6ae5834b ("net: dsa: b53: add MTU configuration support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>