net: ixp4xx_eth: Support changing the MTU

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

After locating an out-of-tree patch in OpenWrt I found
suitable code to set the MTU on the interface and ported
it and updated it. Now the MTU gets set properly.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20231005-ixp4xx-eth-mtu-v4-1-08c66ed0bc69@linaro.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>