ntb: Split ntb_setup_msix() into separate BWD/SNB routines

This is an cleanup effort to make ntb_setup_msix() more
readable - use ntb_setup_bwd_msix() to init MSI-Xs on
BWD hardware and ntb_setup_snb_msix() - on SNB hardware.

Function ntb_setup_snb_msix() also initializes MSI-Xs the
way it should has been done - looping pci_enable_msix()
until success or failure.

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