net/mlx5: Define fdb tc levels per prio

Define FDB_TC_LEVELS_PER_PRIO instead of magic number 2.
This is the number of levels used by each tc prio table in the fdb.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>