net/mlx5: fix Tx doorbell

net/mlx5: fix Tx doorbell

Too much data is uselessly written to the Tx doorbell, which since v16.11
may also cause Tx queues to behave erratically and crash applications.

This regression was seen on VF devices when the BlueFlame buffer size is
zero (txq->bf_buf_size) due to the following change:

 -       txq->bf_offset ^= txq->bf_buf_size;
 +       txq->bf_offset ^= (1 << txq->bf_buf_size);

Fixes: 1d88ba171942 ("net/mlx5: refactor Tx data path")
Fixes: d5793daefec8 ("net/mlx5: reduce memory overhead for BF handling")

Signed-off-by: Nelio Laranjeiro <nelio.laranjeiro@6wind.com>
Acked-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>