net/mlx5_core: Fix caching ATOMIC endian mode capability

Add caching of maximum device capability of ATOMIC endian mode.

Fixes: f91e6d89 ('net/mlx5_core: Add setting ATOMIC endian mode')
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>