net: dsa: b53: fix max MTU for 1g switches

JMS_MAX_SIZE is the ethernet frame length, not the MTU, which is payload
without ethernet headers.

According to the datasheets maximum supported frame length for most
gigabyte swithes is 9720 bytes, so convert that to the expected MTU when
using VLAN tagged frames.

Fixes: 6ae5834b ("net: dsa: b53: add MTU configuration support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/dsa/b53/b53_common.c

@@ -27,6 +27,7 @@
 #include <linux/phylink.h>
 #include <linux/etherdevice.h>
 #include <linux/if_bridge.h>
+#include <linux/if_vlan.h>
 #include <net/dsa.h>
 
 #include "b53_regs.h"
@@ -224,6 +225,8 @@ static const struct b53_mib_desc b53_mibs_58xx[] = {
 
 #define B53_MIBS_58XX_SIZE	ARRAY_SIZE(b53_mibs_58xx)
 
+#define B53_MAX_MTU		(9720 - ETH_HLEN - VLAN_HLEN - ETH_FCS_LEN)
+
 static int b53_do_vlan_op(struct b53_device *dev, u8 op)
 {
 	unsigned int i;
@@ -2267,7 +2270,7 @@ static int b53_change_mtu(struct dsa_switch *ds, int port, int mtu)
 
 static int b53_get_max_mtu(struct dsa_switch *ds, int port)
 {
-	return JMS_MAX_SIZE;
+	return B53_MAX_MTU;
 }
 
 static const struct phylink_mac_ops b53_phylink_mac_ops = {