net: dsa: mv88e6xxx: Fix counting of ATU violations

The ATU port vector contains a bit per port of the switch. The code
wrongly used it as a port number, and incremented a port counter. This
resulted in the wrong interfaces counter being incremented, and
potentially going off the end of the array of ports.

Fix this by using the source port ID for the violation, which really
is a port number.

Reported-by: Chris Healy <Chris.Healy@zii.aero>
Tested-by: Chris Healy <Chris.Healy@zii.aero>
Fixes: 65f60e45 ("net: dsa: mv88e6xxx: Keep ATU/VTU violation statistics")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>