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As we don't specify the MTU in the driver, the framework
will fall back to 1500 bytes and this doesn't work very
well when we try to attach a DSA switch:

  eth1: mtu greater than device maximum
  ixp4xx_eth c800a000.ethernet eth1: error -22 setting
  MTU to 1504 to include DSA overhead

I checked the developer docs and the hardware can actually
do really big frames, so update the driver accordingly.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20230923-ixp4xx-eth-mtu-v1-1-9e88b908e1b2@linaro.org

Signed-off-by: Paolo Abeni <pabeni@redhat.com>