net/mlx5: Make eswitch support to depend on switchdev

Add dependancy for switchdev to be congfigured as any user-space control
plane SW is expected to use the HW switchdev ID to locate the representors
related to VFs of a certain PF and apply SW/offloaded switching on them.

Fixes: e80541ec

 ('net/mlx5: Add CONFIG_MLX5_ESWITCH Kconfig')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>