ixgbevf: Cleanup variable usage, improve stack performance

This change is meant to help cleanup the usage of temporary variables
within the Rx hot-path by removing unnecessary variables and reducing
the scope of variables that do not need to exist outside the main loop.

CC: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>