[POWERPC] pasemi: Add workaround for erratum 5945

Erratum 5945 causes some of the registers on the PCIe root ports to
not read correctly.  Do a small dance to avoid this: Write an unused
register, read the value and write it back.  Thankfully this is not in
a hot code path.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>