net: systemport: set dev->max_mtu to UMAC_MAX_MTU_SIZE

The driver is already allocating receive buffers of 2KiB and the
Ethernet MAC is configured to accept frames up to UMAC_MAX_MTU_SIZE.

Fixes: bfcb813 ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20201218173843.141046-1-f.fainelli@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/bcmsysport.c

@@ -2577,6 +2577,7 @@ static int bcm_sysport_probe(struct platform_device *pdev)
 			 NETIF_F_HW_VLAN_CTAG_TX;
 	dev->hw_features |= dev->features;
 	dev->vlan_features |= dev->features;
+	dev->max_mtu = UMAC_MAX_MTU_SIZE;
 
 	/* Request the WOL interrupt and advertise suspend if available */
 	priv->wol_irq_disabled = 1;