Use htons instead of unconditionally byte swapping nexthdr.  On a little
endian systems shifting the byte is correct behavior, but it results in
incorrect csums on big endian architectures.

Fixes: f8c6455b ('net/mlx4_en: Extend checksum offloading by CHECKSUM COMPLETE')
Signed-off-by: NDaniel Jurgens <danielj@mellanox.com>
Reviewed-by: NCarol Soto <clsoto@us.ibm.com>
Tested-by: NCarol Soto <clsoto@us.ibm.com>
Signed-off-by: NTariq Toukan <tariqt@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>