epl: Reassembly table fix

 Fixed an error that retransmitted frames were added to the
 reassembly table.

Change-Id: I314412cb8f2ce49142e4b7f001613948f5e03bf6
Reviewed-on: https://code.wireshark.org/review/4916
Reviewed-by: Roland Knall <rknall@gmail.com>
Reviewed-by: Lukas Emersberger <lukas.emersberger@gmail.com>
Reviewed-by: Michael Mann <mmann78@netscape.net>