net/mlx5e: Add NAPI statistics

Add per-channel and global ethtool counters for NAPI.
This helps us monitor and analyze performance in general.

- ch[i]_poll:
  the number of times the channel's NAPI poll was invoked.

- ch[i]_arm:
  the number of times the channel's NAPI poll completed
  and armed the completion queues.

- ch[i]_aff_change:
  the number of times the channel's NAPI poll explicitly
  stopped execution on a cpu due to a change in affinity.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>