PCI: thunder: Don't clobber read-only bits in bridge config registers

The 32-bit addressing modes in the I/O and Prefetchable Memory registers
are required to be read-only.  Since the underlying access method allows
them to be set, emulate their read-only nature and always set them.

Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>