net: ixp4xx_eth: Specify min/max 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

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

drivers/net/ethernet/xscale/ixp4xx_eth.c

@@ -24,6 +24,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/dmapool.h>
 #include <linux/etherdevice.h>
+#include <linux/if_vlan.h>
 #include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/net_tstamp.h>
@@ -1488,6 +1489,13 @@ static int ixp4xx_eth_probe(struct platform_device *pdev)
 	ndev->dev.dma_mask = dev->dma_mask;
 	ndev->dev.coherent_dma_mask = dev->coherent_dma_mask;
 
+	/* Maximum frame size is 16320 bytes and includes VLAN and
+	 * ethernet headers. See "IXP400 Software Programmer's Guide"
+	 * section 10.3.2, page 161.
+	 */
+	ndev->min_mtu = ETH_MIN_MTU;
+	ndev->max_mtu = 16320 - VLAN_ETH_HLEN;
+
 	netif_napi_add_weight(ndev, &port->napi, eth_poll, NAPI_WEIGHT);
 
 	if (!(port->npe = npe_request(NPE_ID(port->id))))