app/testpmd: fix Tx VLAN and QinQ dependency

Tx VLAN & QinQ insert enable need not depend on
Rx VLAN offload ETH_VLAN_EXTEND_OFFLOAD. For Tx VLAN
insert enable, error check is now to see if QinQ was enabled
but only single VLAN id is set.

Fixes: 6a34f91690d0 ("app/testpmd: fix error message when setting Tx VLAN")
Cc: stable@dpdk.org
Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>