PCI: Don't clear IORESOURCE_UNSET when updating BAR

Clear IORESOURCE_UNSET when we assign an address to a resource, not when we
write the address to the BAR.

Also, drop the "BAR %d: set to %pR" message; this is mostly redundant with
the "BAR %d: assigned %pR" message from pci_assign_resource().

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>