net/mlx4_en: Set csum level for encapsulated packets

This was dropped by mistake for the napi_gro_frags flow, fix that.

Fixes: dd65beac ('net/mlx4_en: Extend usage of napi_gro_frags')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>