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>

Documentation/MSI-HOWTO.txt

@@ -236,10 +236,8 @@ software system can set different pages for controlling accesses to the
 MSI-X structure. The implementation of MSI support requires the PCI
 subsystem, not a device driver, to maintain full control of the MSI-X
 table/MSI-X PBA (Pending Bit Array) and MMIO address space of the MSI-X
-table/MSI-X PBA.  A device driver is prohibited from requesting the MMIO
-address space of the MSI-X table/MSI-X PBA. Otherwise, the PCI subsystem
-will fail enabling MSI-X on its hardware device when it calls the function
-pci_enable_msix().
+table/MSI-X PBA.  A device driver should not access the MMIO address
+space of the MSI-X table/MSI-X PBA.
 
 5.3.2 API pci_enable_msix