net: dsa: qca8k: Fix port MTU setting

The qca8k only supports a switch-wide MTU setting, and the code to take
the max of all ports was only looking at the port currently being set.
Fix to examine all ports.

Reported-by: DENG Qingfang <dqfext@gmail.com>
Fixes: f58d259 ("net: dsa: qca8k: implement the port MTU callbacks")
Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20201030183315.GA6736@earth.li
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/qca8k.c

@@ -1219,8 +1219,8 @@ qca8k_port_change_mtu(struct dsa_switch *ds, int port, int new_mtu)
 	priv->port_mtu[port] = new_mtu;
 
 	for (i = 0; i < QCA8K_NUM_PORTS; i++)
-		if (priv->port_mtu[port] > mtu)
-			mtu = priv->port_mtu[port];
+		if (priv->port_mtu[i] > mtu)
+			mtu = priv->port_mtu[i];
 
 	/* Include L2 header / FCS length */
 	qca8k_write(priv, QCA8K_MAX_FRAME_SIZE, mtu + ETH_HLEN + ETH_FCS_LEN);