net: dsa: bcm_sf2: Propagate ethtool::rxnfc to CPU port

Allow propagating ethtool::rxnfc programming to the CPU/management port
such that it is possible for such a CPU to perform e.g: Wake-on-LAN
using filters configured by the switch. We need a tiny bit of
cooperation between the switch drivers which is able to do the full flow
matching, whereas the CPU/management port might not. The CPU/management
driver needs to return -EOPNOTSUPP to indicate an non critical error,
any other error code otherwise.

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