net/mlx5: Set command entry semaphore up once got index free

Avoid a race where command work handler may fail to allocate command
entry index, by holding the command semaphore down till command entry
index is being freed.

Fixes: 410bd754 ("net/mlx5: Add retry mechanism to the command entry index allocation")
Signed-off-by: Moshe Shemesh <moshe@nvidia.com>
Reviewed-by: Eran Ben Elisha <eranbe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>