net/mlx5e: Unify slow PCI heuristic

Get the link/pci speed query and logic into a single function.
Unify the heuristics and use a single PCI threshold (16G) for all.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>