bnx2x: prevent GRO false checksum claims

This patch introduces a more robust error handling flow in case of incorrect
behaviour by the FW when passing on GRO aggregations.

Although this should never happen (i.e., this is merely a theoretical fix),
if the bnx2x driver was to receive a GRO from FW with protocol other than
IPv4/IPv6, the driver would falsely claim to have performed partial
checksum and set various incorrect fields in the skb header.

Current behaviour of the bnx2x driver (i.e., print an error) is insufficient.
This patch remedies this by simply preventing the false claims.

Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>