net: dsa: b53: Add support for reading PHY statistics

Allow the b53 driver to return PHY statistics when the CPU port used is
different than 5, 7 or 8, because those are typically PHY-less on most
devices. This is useful for debugging link problems between the switch
and an external host when using a non standard CPU port number (e.g: 4).

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>