net/mlx5: E-Switch, expose eswitch manager vport

Expose the ability the query the eswitch manager vport number.
Next patch will utilize this capability to reveal the correct
register C0 value to the users.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Link: https://lore.kernel.org/r/614fb0e216250e2ce3340471ec141b83ec45c7f4.1701871118.git.leon@kernel.org


Signed-off-by: Leon Romanovsky <leon@kernel.org>