net/mlx5e: Set PTP channel pointer explicitly to NULL

When closing the PTP channel, set its pointer explicitly to NULL. PTP
channel is opened on demand, the code verify the pointer validity before
access. Nullify it when closing the PTP channel to avoid unexpected
behavior.

Fixes: 145e5637 ("net/mlx5e: Add TX PTP port object support")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>