net: dsa: b53: Improve flow control setup on BCM5301x

According to the Broadcom's reference driver flow control needs to be
enabled for any CPU switch port (5, 7 or 8 - depending on which one is
used). Current code makes it work only for the port 5. Use
dsa_is_cpu_port() which solved that problem.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/b53/b53_common.c

@@ -1222,7 +1222,7 @@ static void b53_adjust_link(struct dsa_switch *ds, int port,
 		return;
 
 	/* Enable flow control on BCM5301x's CPU port */
-	if (is5301x(dev) && port == dev->cpu_port)
+	if (is5301x(dev) && dsa_is_cpu_port(ds, port))
 		tx_pause = rx_pause = true;
 
 	if (phydev->pause) {