net/mlx5e: Rx, checksum handling refactoring

Move vlan checksum fixup flow into mlx5e_skb_padding_csum(), which is
supposed to fixup SKB checksum if needed. And rename
mlx5e_skb_padding_csum() to mlx5e_skb_csum_fixup().

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>