net: dsa: loop: Hook PHY statistics

We just return the same statistics through ethtool_get_stats() and
ethtool_get_phy_stats() for simplicity since this is just a mock-up driver.

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