net/mlx5: Simplify mlx5_sriov_is_enabled() by using pci core API

It is desired to get rid of num_vfs stored inside mlx5_core_sriov to
safely support vports more than vfs.
To reduce dependency on mlx5_core_sriov num_vfs, start using
pci_num_vf() from pci core.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Bodong Wang <bodong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>