ioat: kill msix_single_vector support

Once we have determined that we will not have all of our desired msix
vectors there is no point in attempting a single msix allocation.  The
driver will already need to read registers to determine the source of
the interrupt the fact that it is msix is moot.  Fallback directly to
msi.

Reported-by: Alexander Gordeev <agordeev@redhat.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>