IB/mlx5: Introduce and use mlx5_core_is_vf()

Instead of deciding a given device is virtual function or
not based on a device is PF or not, use already defined
MLX5_COREDEV_VF by introducing an helper API mlx5_core_is_vf().

This enables to clearly identify PF, VF and non virtual functions.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Vu Pham <vuhuong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>