net/mlx5: implement descriptor status API

net/mlx5: implement descriptor status API

Since there is no "descriptor done" flag like on Intel drivers, the
approach is different on mlx5 driver.
- for Tx, we call txq_complete() to free descriptors processed by
  the hw, then we check if the descriptor is between tail and head
- for Rx, we need to browse the cqes, managing compressed ones,
  to get the number of used descriptors.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>