IB/mthca: Use pci_request_regions()

Back in prehistoric (pre-git!) days, the kernel's MSI-X support did
request_mem_region() on a device's MSI-X tables, which meant that a
driver that enabled MSI-X couldn't use pci_request_regions() (since
that would clash with the PCI layer's MSI-X request).

However, that was removed (by me!) years ago, so mthca can just use
pci_request_regions() and pci_release_regions() instead of its own
much more complicated code that avoids requesting the MSI-X tables.

Signed-off-by: Roland Dreier <rolandd@cisco.com>