enic: be less verbose about non-critical firmware errors

If a feature is not supported by firmware no need to print an error message.
This surpresses the following harmless message on boot up and ethtool query.
enic: Error 1 devcmd 36

Signed-off-by: Stefan Assmann <sassmann@kpanic.de>
Signed-off-by: David S. Miller <davem@davemloft.net>