By design, async enqueue API should return directly if async device
is not registered. This patch removes the corrupted implementation of
the enqueue fallback from async mode to sync mode.
Fixes: cd6760da1076 ("vhost: introduce async enqueue for split ring")
Cc: stable@dpdk.org
Signed-off-by: Patrick Fu <patrick.fu@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>