net/mlx5_core: Refactor device capability function

Device capability function was called similar in all places.
It was called twice for every queried parameter, while the
difference between calls was in HCA capability mode only.

The change proposed unify these calls into one function.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>