net: dsa: Discard frames from unused ports

commit fc5f33768cca7144f8d793205b229d46740d183b upstream.

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.

This has been reworked for 4.14, which does not have the central
dsa_master_find_slave() function, so each tag driver needs to check.

Reported-by: Chris Healy <cphealy@gmail.com>
Fixes: 91da11f8 ("net: Distributed Switch Architecture protocol support")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>