Fix checks done when changing generator pkt_size

Fix checks done when changing generator pkt_size

Different checks were done when setting the generator pkt_size.
In case of wrong pkt_size (e.g. too big) an error was printed but
the pkt_size was still set causing a potential corruption.
In addition, in case of a packet size incompatible with some
of the packet fields (e.g. latency data) PROX was panicing
while this should not happen runtime for such an error.

Change-Id: Ifa11475bf295aaac7b0255c1bf9b5feed8ef90c4
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>