mlx5e: Fix sparse warnings in mlx5e_handle_csum().

>> drivers/net/ethernet/mellanox/mlx5/core/en_rx.c:173:44: sparse: incorrect type in argument 1 (different base types)
   drivers/net/ethernet/mellanox/mlx5/core/en_rx.c:173:44:    expected restricted __sum16 [usertype] n
   drivers/net/ethernet/mellanox/mlx5/core/en_rx.c:173:44:    got restricted __be16 [usertype] check_sum

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>