ethdev: fix variable length flow elements support

ethdev: fix variable length flow elements support

RTE flow API defines two flow elements types - common and PMD private.
Common RTE flow types are defined in rte_flow.h while PMD private
types exists inside specific PMD only. Application can create a flow
rule with PMD private items or actions. RTE flow API restricts
private PMD types to negative values.

Current implementation tried to use negative PMD private item type
value as index in the rte_flow_desc_item[] array.

The patch allows access to rte_flow_desc_item[] and
rte_flow_desc_action[] arrays to non-private PMD types only.

Fixes: 6cf72047332b ("ethdev: support flow elements with variable length")
Signed-off-by: Gregory Etelson <getelson@nvidia.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>