PCI: pciehp: Use RMW accessors for changing LNKCTL

As hotplug is not the only driver touching LNKCTL, use the RMW capability
accessor which handles concurrent changes correctly.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Fixes: 7f822999 ("PCI: pciehp: Add Disable/enable link functions")
Link: https://lore.kernel.org/r/20230717120503.15276-4-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>