net/mlx5e: Split SW group counters update function

SW group counter update function aggregates sw stats out of many
mlx5e_*_stats resides in a given mlx5e_channel_stats struct.
Split the function into a few helper functions.

This will be used later in the series to calculate specific
mlx5e_*_stats which are not defined inside mlx5e_channel_stats.
Signed-off-by: Eran Ben Elisha <eranbe@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>