net/mlx5e: Use pcie_bandwidth_available() to compute bandwidth
Use the new pci_bandwidth_available() function to calculate maximum
available bandwidth through the PCI chain instead of computing it ourselves
with mlx5e_get_pci_bw().
This is used to detect when the device is capable of more bandwidth than is
available in the current slot. The driver may adjust compression settings
accordingly.
Note that pci_bandwidth_available() accounts for PCIe encoding overhead, so
it is more accurate than mlx5e_get_pci_bw() was.