net/mlx5: rework hardware structures

Rework Work Queue Element (aka WQE) structures to fit PMD needs.
A WQE is an aggregation of 16 bytes elements known as "data segments"
(aka dseg).

The only common part is the first two elements i.e. the control one to
define the job type, and the Ethernet segment which embed offload requests
with other information, after that, it can have:
  - a raw data packet,
  - a data pointer to the packet itself,
  - both.

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>