PCI: fix MSI-HOWTO.txt info about MSI-X MMIO space

The current MSI-HOWTO.txt says that device drivers should not request the
memory space that contains MSI-X tables.  This is because the original
MSI-X implementation did a request_mem_region() on this space, but that
code was removed long ago (in the pre-git era, in fact).  Years after the
code was changed, we might as well clean up the documention to avoid a
confusing mention of requesting regions: drivers using MSI-X can just use
pci_request_regions() just like any other driver, and so there's no need
for MSI-HOWTO.txt to talk about this at all.

Signed-off-by: Roland Dreier <roland@digitalvampire.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>