PCI: Cleanup PCI_REBAR_CTRL_BAR_SHIFT handling

Cleanup PCI_REBAR_CTRL_BAR_SHIFT handling.  That was hard coded instead of
properly defined in the header for some reason.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>