PCI: Disable decoding for BAR sizing only when it was actually enabled

We disable BARs while sizing them so we don't cause conflicts with other
devices (see 253d2e54 and bbffe435

).  But if device decoding is already
disabled before we size the BAR, we don't need to disable it again.

[bhelgaas: changelog, add PCI_COMMAND_DECODING_ENABLE for readability]
Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>