Finish renaming si_code to mtp3_si_code.

I5e183b997768c87f275569343afa69ca542aba46 missed a spot.

Change-Id: I0970f7e50719e71efc824bbb07a6c2a69c66e991
Reviewed-on: https://code.wireshark.org/review/5845
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>