ntb: Fix missed call to pci_enable_msix()

Current MSI-X enablement code assumes MSI-Xs were successfully
allocated in case less than requested vectors were available.
That assumption is wrong, since MSI-Xs should be enabled with
a repeated call to pci_enable_msix(). This update fixes this.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Jon Mason <jon.mason@intel.com>