net/mlx5: fix modify field action order for IPv6

net/mlx5: fix modify field action order for IPv6

Mellanox hardware can only modify any packet field in 32-bit chunks,
which means 4 such chunks are needed to modify an IPv6 address.
The modification order of these chunks starts from the most significant
bits for the IPv6 address. That leads to confusing results when trying
to modify either source or destination address via the MODIFY_FIELD
action. Fix the order of 32-bit chunks for IPv6 addresses modification
by starting from the least significant bits.

Fixes: 641dbe4fb053 ("net/mlx5: support modify field flow action")
Cc: stable@dpdk.org
Signed-off-by: Alexander Kozyrev <akozyrev@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>