net: dsa: bcm_sf2: Allow looping back CFP rules

When the source and destination port of a CFP rule match, we must set
the loopback bit enable to allow that, otherwise the frame is discarded.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>