net/mlx4_en: Don't configure the HW vxlan parser when vxlan offloading isn't set

The add_vxlan_port ndo driver code was wrongly testing whether HW vxlan offloads
are supported by the device instead of checking if they are currently enabled.

This causes the driver to configure the HW parser to conduct matching for vxlan
packets but since no steering rules were set, vxlan packets are dropped on RX.

Fix that by doing the right test, as done in the del_vxlan_port ndo handler.

Fixes: 1b136de1 ('net/mlx4: Implement vxlan ndo calls')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>