net/mlx5: Set number of allowed levels in priority

Refactors the flow steering namespace creation,
by changing the name num_fts to num_levels.
When new flow table is created, the driver assign new level
to this flow table therefore the meaning is equivalent.
Since downstream patches will introduce the ability to create more
than one flow table per level, the name num_fts is no
longer accurate.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>