net/mlx5e: IPoIB, Fix RX checksum statistics update

Update the RX checksum only if the feature is enabled.

Fixes: 9d6bd752 ("net/mlx5e: IPoIB, RX handler")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>