net/mlx5: fix flex flow item header length

net/mlx5: fix flex flow item header length

There is the updated firmware providing the new capability
bit "header_length_field_offset_mode". If this bit is set
the length field offset in flex parser configuration should
not be adjusted by the supported field mask left margin,
and the bit "header_length_field_offset_mode" should be set
in configuration command on the flex parser creation
firmware call.

Fixes: b04b06f4cb3f ("net/mlx5: fix flex item header length field translation")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>