Fix using signature in latency measurements

Fix using signature in latency measurements

When doing latency measurements the generator can add a
32 bits signature in the packet at a specific location,
so that the receiver only uses the packets generated by the
generator and ignores packets generated for instance by a switch
The is particuly important for latency measurements as we use
data in the packets as timestamps, and packets generated elsewhere
would result in random latency for those packets.

Change-Id: I8352b35aff76ec8d1344a1e492b9dcc20a53f1ce
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>