lantiq: xrx200: enable use of baby jumbo frames

lantiq: xrx200: enable use of baby jumbo frames

xrx200 max MTU is reduced so that it works correctly when set to the
max, and the max MTU of the switch is increased to match.

In 5.10, the switch driver now 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 used, this should have no effect. The maximum packet size is
limited as large packets cause the switch to lock up.

0702-net-lantiq-add-support-for-jumbo-frames.patch comes from net-next
commit 998ac358019e491217e752bc6dcbb3afb2a6fa3e.

In 5.4, all switch ports are configured to accept the max MTU, as 5.4
does not have port_max_mtu/port_change_mtu callbacks.

Signed-off-by: Thomas Nixon <tom@tomn.co.uk>