IB/mthca: Use pci_enable_msix_exact() instead of pci_enable_msix()

As result of the deprecation of the MSI-X/MSI enablement functions
pci_enable_msix() and pci_enable_msi_block(), all drivers using these
two interfaces need to be updated to use the new
pci_enable_msi_range() or pci_enable_msi_exact() and
pci_enable_msix_range() or pci_enable_msix_exact() interfaces.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>