net/mlx5: fix Rx descriptor status

net/mlx5: fix Rx descriptor status

Three bugs in rx_queue_count function:
- One entry may contain several segments, so 'used' must be multiplied
  by number of segments per entry to properly reflect the queue usage.
- The number of cqes is equals to (1U << rxq->elts_n) - 1 in SPRQ mode.
  The range returned by rx_queue_count should be the number of entries
  used in queue, so it ranges from 0 to max number of entries
  in queue, not this number minus one.
- For MPRQ mode, we need to take into account of the number of strd.

Fixes: 8788fec1f269 ("net/mlx5: implement descriptor status API")
Cc: stable@dpdk.org
Signed-off-by: Didier Pallard <didier.pallard@6wind.com>
Signed-off-by: Maxime Leroy <maxime.leroy@6wind.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>