net/vhost: fix TSO feature default disablement

net/vhost: fix TSO feature default disablement

By default, TSO feature should be disabled because it requires
application's support to be functional as mentioned in the
documentation.

However, if "tso" devarg was not specified, the feature did
not get disabled.

This patch fixes this issue, so that TSO is disabled, even if
"tso=0" is not passed as devarg.

Fixes: e289400669d5 ("net/vhost: support TSO disabling")
Cc: stable@dpdk.org
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>