nfp: update the old flash error message

Apparently there are still cards in the wild with a very old
management FW.  Let's make the error message in that case
indicate more clearly that management firmware has to be
updated.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>