net/mlx4_en: RX csum, pre-define enabled protocols for IP status masking

Pre-define a mask for IP status of a completion, that tests the
MLX4_CQE_STATUS_IPV6 only in case CONFIG_IPV6 is enabled.
Use it for IP status testing upon completion, instead of separating
the datapath into two flows.
This takes common code structures (such as closing parenthesis)
back to their original place, and makes code more readable.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>