net/mlx5e: Fix enabling sriov while tc nic rules are offloaded

There is a total of four 4M entries flow tables. In sriov disabled
mode, ct, ct_nat and post_act take three of them. When adding the
first tc nic rule in this mode, it will take another 4M table
for the tc <chain,prio> table. If user then enables sriov, the legacy
flow table tries to take another 4M and fails, and so enablement fails.

To fix that, have legacy fdb take the next available maximum
size from the fs ft pool.

Fixes: 4a98544d ("net/mlx5: Move chains ft pool to be used by all firmware steering")
Signed-off-by: Paul Blakey <paulb@nvidia.com>
Reviewed-by: Roi Dayan <roid@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>