net/mlx5: Cap the maximum flow group size to 16M entries

The maximum number of large flow groups applies to both small and large
tables. For very large tables (such as the 2G SW steering tables) this may
create a small number of flow groups each with an unrealistic entries
domain (> 16M).

Set the maximum number of large flow groups to at least what user
requested, but with a maximum per group size of 16M entries.
For software steering, if user requested less than 128 large flow
groups, it will gives us about 128 16M groups in a 2G
entries tables.

Signed-off-by: Paul Blakey <paulb@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>