vmxnet3: fix default Tx configuration

vmxnet3: fix default Tx configuration

Since commit fbde27f19ab8f "get default Rx/Tx configuration from dev info",
a default RX/TX configuration can be used for all PMDs.
In case of vmxnet3, the whole structure was zeroed and not filled out.
The PMD does not support multi segments or offload functions,
so txq_flags should have those flags set.

Test report: http://dpdk.org/ml/archives/dev/2014-December/009933.html

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Tested-by: Xiaonan Zhang <xiaonanx.zhang@intel.com>