ptxed: only compute stats when requested

Statistics are currently computed whether or not --stat is specified.  The
option only affects printing statistics at the end.

Do not compute stats if we don't want to print them.

Change-Id: I77e8d6ab4be60b62e706714e6429d1759b83eb44
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>