test/ring: fix object reference for single element enqueue

test/ring: fix object reference for single element enqueue

When enqueue one element to ring in the performance test, a pointer
should be passed to rte_ring_[sp|mp]enqueue APIs, not the pointer
to a table of void *pointers.

Fixes: a9fe152363e2 ("test/ring: add custom element size functional tests")
Cc: stable@dpdk.org
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>