net: dsa: Discard frames from unused ports

net: dsa: Discard frames from unused ports

The Marvell switches under some conditions will pass a frame to the
host with the port being the CPU port. Such frames are invalid, and
should be dropped. Not dropping them can result in a crash when
incrementing the receive statistics for an invalid port.

Reported-by: Chris Healy <[email protected]>
Fixes: 91da11f870f0 ("net: Distributed Switch Architecture protocol support")
Signed-off-by: Andrew Lunn <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>