Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - ixia: Fix IXIA in trafficgen mode

    There are two IXIA wrappers in VSPERF - IxNET and Ixia. It has
    been found that legacy Ixia implementation is no longer working
    in trafficgen mode. This issues has been fixed.

    JIRA: VSPERF-513

    Change-Id: Ief1db71f688a19c5b41508617ccbc73024a02c48
    Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index 862ef28..0975aa2 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit 862ef28b808aca4dd75cc44a83023362dcce1969
+Subproject commit 0975aa2ec9baf2e852be7991f62ba40865997456