net/mlx4_en: RX csum, remove redundant branches and checks

Do not check IPv6 bit in cqe status if CONFIG_IPV6 is not enabled.
Function check_csum() is reached only with IPv4 or IPv6 set (if enabled),
if IPv6 is not set (or is not enabled) it is redundant to test the
IPv4 bit.

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