ethdev: return named opaque type instead of void pointer

ethdev: return named opaque type instead of void pointer

"struct rte_eth_rxtx_callback" is defined as internal data structure and
used as named opaque type.

So the functions that are adding callbacks can return objects in this
type instead of void pointer.

Also const qualifier added to "struct rte_eth_rxtx_callback *" to
protect it better from application modification.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>