Set VLAN action mode to VLAN_REWRITE only if VLAN_INSERT has not been
set yet. Otherwise, the resulting VLAN packets will have their VLAN
header rewritten, instead of pushing a new outer VLAN header.
Also fix the VLAN ID extraction logic and endianness issues.
Fixes: 1decc62b1cbe ("net/cxgbe: add flow operations to offload VLAN actions")
Cc: stable@dpdk.org
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>