PCI: pci-bridge-emul: Fix Root Cap/Status comment

The upper 16-bits of Root Control contain the Root Capabilities
register. The code instead describes the Root Status register in the
upper 16-bits, although it uses the correct bit definition for Root
Capabilities, and for Root Status in the next definition.

Fix this comment and add a comment describing the Root Status register.

Link: https://lore.kernel.org/r/20200511162117.6674-3-jonathan.derrick@intel.com


Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>