nfp: flower: check for/turn on LAG support in firmware

Check if the fw contains the _abi_flower_balance_sync_enable symbol. If it
does then write a 1 to this indicating that the driver is willing to
receive NIC to kernel LAG related control messages.

If the write is successful, update the list of extra features supported by
the fw and add a stub to accept LAG cmsgs.

Signed-off-by: John Hurley <john.hurley@netronome.com>
Reviewed-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>