net/mlx5: fix metadata endianness in modify field action

net/mlx5: fix metadata endianness in modify field action

As modify field action immediate source parameter the metadata
should follow the CPU endianness (according to SET_META action
structure format), and mlx5 PMD wrongly handled the immediate
parameter metadata buffer as big-endian, resulting in wrong
metadata set action with incorrect endianness.

Fixes: 40c8fb1fd3b3 ("net/mlx5: update modify field action")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>