JMS_MIN_SIZE is the full ethernet frame length, while mtu is just the
data payload size. Comparing these two meant that mtus between 1500 and
1518 did not trigger enabling jumbo frames.

So instead compare the set mtu ETH_DATA_LEN, which is equal to
JMS_MIN_SIZE - ETH_HLEN - ETH_FCS_LEN;

Also do a check that the requested mtu is actually greater than the
minimum length, else we do not need to enable jumbo frames.

In practice this only introduced a very small range of mtus that did not
work properly. Newer chips allow 2000 byte large frames by default, and
older chips allow 1536 bytes long, which is equivalent to an mtu of
1514. So effectivly only mtus of 1515~1517 were broken.

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>