mlx5: fix get_ip_proto()

IP header is not necessarily located right after struct ethhdr,
there could be multiple 802.1Q headers in between, this is why
we call __vlan_get_protocol().

Fixes: fe1dc069 ("net/mlx5e: don't set CHECKSUM_COMPLETE on SCTP packets")
Cc: Alaa Hleihel <alaa@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>