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>