PCI: Rename resource variable from r to res

Resource is going to be passed in as argument aften an upcoming change.
Rename the struct resource variable from "r" to "res" to avoid using one
letter variable name in a function argument.

This rename is made separately to reduce churn in the upcoming change.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250829131113.36754-18-ilpo.jarvinen@linux.intel.com