ixgbevf: Update Rx next to clean in real time

Since the next_to_clean value is only accessed by the Rx interrupt handler
we can save on stack space by just storing our updated values back in
next_to_clean instead of using the stack variable i.  This should help to
reduce stack space and we can further collapse the size of the function.

Also removed  non_eop_descs counter as it was never shown in the stats.

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>