net: mvpp2: Only clear the stat counters at port init

When first configuring a port on PPv2, we want to clear the internal
counters so that we don't get values from previous boot stages.

However, we can't really clear these counters when resetting the MAC,
since there are valid reasons to do so while the port is being used,
such as when reconfiguring the interface mode with the PHY.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>