PCI: Make the "entries" argument to pci_enable_msix() optional

The "entries" argument isn't needed if the list of entries does not contain
any holes.  Make it optional so that we can avoid the need to allocate a
msix_entry structure for this (common) case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Alexander Gordeev <agordeev@redhat.com>