net: dsa: lantiq_gswip: enable jumbo frames on GSWIP

net: dsa: lantiq_gswip: enable jumbo frames on GSWIP

This enables non-standard MTUs on a per-port basis, with the overall
frame size set based on the CPU port.

When the MTU is not changed, this should have no effect.

Long packets crash the switch with MTUs of greater than 2526, so the
maximum is limited for now. Medium packets are sometimes dropped (e.g.
TCP over 2477, UDP over 2516-2519, ICMP over 2526), Hence an MTU value
of 2400 seems safe.

Signed-off-by: Thomas Nixon <tom@tomn.co.uk>
Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://lore.kernel.org/r/20220308230457.1599237-1-olek2@wp.pl
Signed-off-by: Jakub Kicinski <kuba@kernel.org>