PCI: vmd: Fix config addressing when using bus offsets

VMD maps child device config spaces to the VMD Config BAR linearly
regardless of the starting bus offset. Because of this, the config
address decode must ignore starting bus offsets when mapping the BDF to
the config space address.

Fixes: 2a5a9c9a ("PCI: vmd: Add offset to bus numbers if necessary")
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: stable@vger.kernel.org # v5.2+