net: dsa: b53: Include IMP/CPU port in dumb forwarding mode

Since Broadcom tags are not enabled in b53 (DSA_PROTO_TAG_NONE), we need
to make sure that the IMP/CPU port is included in the forwarding
decision.

Without this change, switching between non-management ports would work,
but not between management ports and non-management ports thus breaking
the default state in which DSA switch are brought up.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Reported-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>