net/mlx5: Expose eswitch encap mode

Add API to get the current Eswitch encap mode.
It will be used in downstream patches to check if
flow table can be created with encap support or not.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>