net: bcmgenet: use promisc for unsupported filters

Currently we silently ignore filters if we cannot meet the filter
requirements. This will lead to the MAC dropping packets that are
expected to pass. A better solution would be to set the NIC to promisc
mode when the required filters cannot be met.

Also correct the number of MDF filters supported. It should be 17,
not 16.

Signed-off-by: Justin Chen <justinpopo6@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>