nfp: track link state changes

For caching link settings - remember if we have seen link events
since the last time the eth_port information was refreshed.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>