net/mlx5: Fix query E-Switch capabilities

E-Switch capabilities should be queried only if E-Switch flow table
is supported and not only when vport group manager.

Fixes: d6666753

 ("net/mlx5: E-Switch, Introduce HCA cap and E-Switch vport context")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>