net: ethernet: ti: am65-cpsw: Fix max mtu to fit ethernet frames

The value of AM65_CPSW_MAX_PACKET_SIZE represents the maximum length
of a received frame. This value is written to the register
AM65_CPSW_PORT_REG_RX_MAXLEN.

The maximum MTU configured on the network device should then leave
some room for the ethernet headers and frame check. Otherwise, if
the network interface is configured to its maximum mtu possible,
the frames will be larger than AM65_CPSW_MAX_PACKET_SIZE and will
get dropped as oversized.

The switch supports ethernet frame sizes between 64 and 2024 bytes
(including VLAN) as stated in the technical reference manual, so
define AM65_CPSW_MAX_PACKET_SIZE with that maximum size.

Fixes: 93a76530 ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
Signed-off-by: Jorge Sanjuan Garcia <jorge.sanjuangarcia@duagon.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Link: https://lore.kernel.org/r/20240105085530.14070-2-jorge.sanjuangarcia@duagon.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/ti/am65-cpsw-nuss.c

@@ -56,7 +56,7 @@
 #define AM65_CPSW_MAX_PORTS	8
 
 #define AM65_CPSW_MIN_PACKET_SIZE	VLAN_ETH_ZLEN
-#define AM65_CPSW_MAX_PACKET_SIZE	(VLAN_ETH_FRAME_LEN + ETH_FCS_LEN)
+#define AM65_CPSW_MAX_PACKET_SIZE	2024
 
 #define AM65_CPSW_REG_CTL		0x004
 #define AM65_CPSW_REG_STAT_PORT_EN	0x014
@@ -2244,7 +2244,8 @@ am65_cpsw_nuss_init_port_ndev(struct am65_cpsw_common *common, u32 port_idx)
 	eth_hw_addr_set(port->ndev, port->slave.mac_addr);
 
 	port->ndev->min_mtu = AM65_CPSW_MIN_PACKET_SIZE;
-	port->ndev->max_mtu = AM65_CPSW_MAX_PACKET_SIZE;
+	port->ndev->max_mtu = AM65_CPSW_MAX_PACKET_SIZE -
+			      (VLAN_ETH_HLEN + ETH_FCS_LEN);
 	port->ndev->hw_features = NETIF_F_SG |
 				  NETIF_F_RXCSUM |
 				  NETIF_F_HW_CSUM |