net: macb: increase max_mtu for oversized frames

Increase max_mtu from 1500 to 1518 bytes when not configured for jumbo
frames. Use 1536 as a starting point as documented in macb.h for
oversized (big) frames, which is the configuration applied in case
jumbo frames capability is not configured; ref. macb_main.c.
Signed-off-by: Pieter Van Trappen <pieter.van.trappen@cern.ch>
Link: https://patch.msgid.link/20240812090657.583821-1-vtpieter@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/cadence/macb_main.c

@@ -5119,12 +5119,12 @@ static int macb_probe(struct platform_device *pdev)
 		goto err_out_free_netdev;
 	}
 
-	/* MTU range: 68 - 1500 or 10240 */
+	/* MTU range: 68 - 1518 or 10240 */
 	dev->min_mtu = GEM_MTU_MIN_SIZE;
 	if ((bp->caps & MACB_CAPS_JUMBO) && bp->jumbo_max_len)
 		dev->max_mtu = bp->jumbo_max_len - ETH_HLEN - ETH_FCS_LEN;
 	else
-		dev->max_mtu = ETH_DATA_LEN;
+		dev->max_mtu = 1536 - ETH_HLEN - ETH_FCS_LEN;
 
 	if (bp->caps & MACB_CAPS_BD_RD_PREFETCH) {
 		val = GEM_BFEXT(RXBD_RDBUFF, gem_readl(bp, DCFG10));