Currently the mlx5e driver has multiple groups of stats, each group is
used for different purposes and it may depend on hardware capabilities
or not. The problem with the current implementation is that there is no
clear API to create a new group of stats.

This change define a new API to create a group of stats and simplifies
the way of handling them by defining a new struct "mlx5e_stats_grp" which
have the following three function pointers:
- get_num_stats() - return the number of counters in the group.
- fill_strings() - fill counters strings within the group.
- fill_stats() - fill counters values within the group.

The above function pointers are used within the ethtool callbaks while
calling "ethtool -S" from userspace. This change also switch the SW
group to use the new API.
Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Reviewed-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>