net/mlx5: Move table size calculation to steering cmd layer

Currently the table size is calculated by the fs_core layer. However, each
steering cmd instance has a different allocation logic. FW steering uses
a predefined pools of multiple sizes. SW steering doesn't have a pool,
and can allocate any size of tables.

Move the table size calculation to the steering cmd layer as a pre-step
for moving fs_chains pool logic globally to firmware steering, and
increasing table sizes for software steering. In addition, change the
size parameter to absolute size to allow the special zero value to
mean "get next available maximum size".

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