net/mlx5e: Statify function mlx5e_monitor_counter_arm

Function usage is limited to the monitor_stats.c file, do not expose it.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>