net/mlx5e: Rename misleading skb_pc/cc references in ptp code

The 'skb_pc/cc' naming is misleading as the values hold the
producer/consumer indices (masked values), not the counters. Rename to
'skb_pi/ci'.

Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Adham Faris <afaris@nvidia.com>