net/mlx5e: Hide function mlx5e_num_channels_changed

No calls for mlx5e_num_channels_changed() out of en_main.c,
turn it static and remove from header.
Keep the wrapper function mlx5e_num_channels_changed_ctx exposed.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Aya Levin <ayal@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>