net/mlx5: Fix turn-off PPS command

Fix a bug of uninitialized pin index when trying to turn off PPS out.

Fixes: de19cd6c

 ("net/mlx5: Move some PPS logic into helper functions")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Eran Ben Elisha <eranbe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>