net/mlx5: fix flex item header length translation

net/mlx5: fix flex item header length translation

Flex item API provides support for network header with a fixed and
variable lengths.
When PMD compiles a new flex item object configuration it converts
RTE parameters into matching PMD PARSE_GRAPH parameters and checks
the parameter values against port capabilities.

Current implementation mismatched PARSE_GRAPH configuration fields
for the fixed size header.

Fixes: b293e8e49d78 ("net/mlx5: translate flex item configuration")
Cc: stable@dpdk.org
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>