ixgbe: count q_vectors instead of MSI-X vectors

It makes much more sense for us to count q_vectors instead of MSI-X
vectors.  We were using num_msix_vectors to find the number of q_vectors in
multiple places.  This was wasteful since we only had one place that
actually needs the number of MSI-X vectors and that is in slow path.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>