examples/l2fwd-crypto: fix option -t

examples/l2fwd-crypto: fix option -t

passing -t 0 as a command line argument causes the application
to exit with an "invalid refresh period specified" error  which is
contrary to applications help text.

This patch removes the unnecessary option "--no-stats" and fixes the
behaviour of the -t parameter.

Fixes: 387259bd6c67 ("examples/l2fwd-crypto: add sample application")
Reported-by: Min Cao <min.cao@intel.com>
Signed-off-by: Declan Doherty <declan.doherty@intel.com>