net/mlx5: Reuse exported virtfn index function call

Instead open-code iteration to compare virtfn internal index, use newly
introduced pci_iov_vf_id() call.

Link: https://lore.kernel.org/all/20220224142024.147653-3-yishaih@nvidia.com
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>