PCI: Use pci_find_vsec_capability() when looking for TBT devices

Currently set_pcie_thunderbolt() open-codes pci_find_vsec_capability().
Refactor the former to use the latter. No functional change intended.

Link: https://lore.kernel.org/r/20211115112902.24033-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>