gro: Name the GRO result enumeration type

This clarifies which return and parameter types are GRO result codes
and not RX result codes.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>